Class 9 Physics - Force And Laws Of Motion

Selvi placed her purse on the passengers seat of her car when she drove to work. By the time she reached her office, her purse had fallen on the floor in front of the passengers seat. Why did this happen? Explain.

According to Newton's first law, a body tends to continue in its state of rest or of uniform motion along a straight line, unless an external force acts on it.

When Selvi applied brakes to stop the car, her purse being in a state of motion tends to continue to be in that state due to inertia of motion, and falls off the passenger seat.

A shopping cart has a mass of $65\ kg$ . In order to accelerate the cart by $0.3\ ms^{-2}$ what force would you exert on it?

Given: Mass of the cart

$m=65\ kg$ , acceleration of the cart

$a=0.3\ m/s^2$

From Newton's second law of motion,

Force

$F = ma$

$\therefore F = 65\times 0.3 = 19.5\ N$

What is the force called which produces an acceleration of $1m{s}^{2}$ in a body of mass $1kg$ ?

Using the formula

$Force = mass \times acceleration$

$Force=(1kg)\times (1 m/s^2)$

$Force=1 kg-m/s^2= 1N$

Hence,

$1 N$ is the force which produces an acceleration of

$1 \ ms^{-2}$ in a body of mass

$1 \ kg$ .

Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. It is based on which law?

it is based on newtons 3rd law for every action there is an equal and opposite reaction

as the water comes out of the sprinkler an equal and opposites force comes in to play hence the sprinkler starts rotating

State Newton's second law of motion.

Newton’s second law of motion gives brief description about magnitude of force from change in momentum.

It stats that: “The force imposed on body is equal to time rate of change in momentum.”

So,

$F = \dfrac{dp}{dt}$

Why is it easier to maintain the motion than to start it?

One has to come over the inertia of rest while starting the motion and inertia of momentum/motion to stop the body which requires maintaining a certain amount of acceleration but to maintain a motion we don't have to apply force as there would not be any acceleration for uniform velocity. Hence it is always easier to maintain the motion than to start or stop the motion.

Do we apply a force on the ground while walking? (Yes/No)

Newton's third law states that every action has an equal and opposite reaction.

This is relevant to walking because when you put your foot on the ground, you are applying a force to it. In doing this, the ground also actually applies an equal force onto your foot, in the opposite direction, pushing you forward.

Here, you are applying force on the ground in the backward direction, and the ground applies forces on your foot in the forward direction which helps you to walk in the forward direction.

To move a boat ahead in water, the boatman has to push the water backwards by his oar. Explain.

According to Newton's third law of motion, every action has equal and opposite reaction. So if boatman wants to move the boat ahead in water, he has to push the water backwards. By pushing the water, boatman applies a force on water in backward direction. In turn, water applies the same amount of force in forward direction, moving the boat ahead.

Newton's Third Law says that every action has an equal and opposite reaction.

State True or False.

Newton's Third Law says that every action has an equal and opposite reaction.

This law is all about how forces on objects balance each other, whether the objects are moving or not. A force acting on an object in one direction is always accompanied by an equal force acting in the opposite direction on the other body.

A heavy car A of mass 2000 kg travelling at 10 m/s has a head-on collision with a sports car B of mass 500 kg.If both cars stop dead on colliding,what was the velocity of car B?

Mass of car A,

$m_1=2000 kg$
Speed of car A,

$v_1=10 m/s$
Mass of car B,

$m_2=500 kg$
Speed of car B,

$v_2$
Acc to law of conservation of momentum

$m_1v_1=m_2v_2$

$2000\times 10=500 \times v_2$

$v_2=\dfrac{2000\times 10}{500}=40 m/s$

Explain why,when a fireman directs a powerful stream of water on a fire from a hose pipe, the hose pipe tends to go backward.

When a fireman directs a powerful stream of water on a fire,the hose pipe tends to go backward due to the reaction force of the water rushing through it in the forward direction at a great speed.

Differentiate between Mass and inertia

Mass

1. Mass is the quantity of
matter in a body regardless
of its volume or of any forces acting on it.

2. Mass is an absolute quantity and is a measure of inertia.

Inertia

1. Inertia is a tendency of an object to continue

its state of rest or motion.

2. It is the resistance of an object to change its state of motion.

3. Inertia depends on mass. Larger the mass, larger the inertia

Give reason: Sparks produced in a grinding stone move tangentially.

Grinding stone is like a wheel and at every point of the wheel at the periphery, the direction of linear velocity is tangential due to the inertia of motion, therefore, the sparks go tangentially.

A bullet of mass $50\ g$ moving with an initial velocity of $100\ m/s$ , strikes a wooden block and comes to rest penetrating a distance $2\ cm$ in it. Calculate the initial momentum of the bullet.

Given,

Mass off the bullet,

$m=50\ g=0.05\ kg$

Initial velocity,

$u = 100\ m/s$

Final velocity,

$v = 0\ m/s$

Distance penetrated,

$s = 2\ cm=0.02\ m$

Initial momentum of the bullet,

$p_{initial}= mu$

$p_{initial}= 0.05 \times 100$

$p_{initial} = 5\ kg\ m/s$

A motorcar with mass $1200kg$ can be pushed by three persons to produce an acceleration of $0.2ms^{-2}$ .With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort).

Mass of motor car

$m = 1200 kg$ .

Acceleration produced

$a = 0.2 ms^{-2}$

Newton second law

$Force(F)= Mass(m)\times acceleration(a)$

Therefore force applied on the car by three persons =Total force

$F$

$= 1200 * 0.2 = 240 N$ .

$\therefore$ Force applied on the car by one person

$=\frac{240}{3} = 80 N$ .

Find out reasons for the situation:
Place a small brick on a plank. When the plank is pulled suddenly the brick remains in the same position as before.

The tendency to resist change in state of motion or rest is called inertia. It is due to this tendency of the brick that the brick remains in the same position as before.

You are hurt when you kick a stone. Why?

This happens due to Newton's third law of motion.

To every action, there is always an equal and opposite reaction. When you kick a stone, you apply some force to it.

To maintain the law of conservation of momentum, the stone also exerts a force on the leg, which we feel as pain.

A body of mass $25 kg$ has a momentum of $125 kg m/s$ . Calculate the velocity of body

Mass of the body,m=25 kg
Momentum p=125 kg.m/s

$p=m \times v$
Mass of the body,

$m=25 kg$
Momentum

$p=125 kg.m/s$ ,

$p= m \times v$

$v=\dfrac{p}{m}=\dfrac{125}{25}=5 m/s$
Velocity of the body is 5 m/s

There are two types of forces X and Y.The forces belonging to type X can produce motion in a stationary object but cannot change the shape of the object .On the other hand,forces belonging to type Y cannot produce motion in stationary object but can change the shape of the object.What is the general name of the forces such as (a) X and (b) Y ?

a) X are unbalanced force.
b) Y are balanced force.

On what factor does the inertia of a body depend? Which has more inertia, a cricket ball or rubber of the same size?

The inertia of a body depends on its mass. A cricket ball has more inertia than a rubber ball of the same size because it has more mass than the rubber ball.

If action is always equal to the reaction, explain how a horse can pull a cart.

Explanation :

$\bullet$ When the horse walks in the forward direction (with the cart attached to it), it exerts a force in the backward direction on the Earth.

$\bullet$ An equal force in the opposite direction (forward direction) is applied on the horse by the earth. This force moves the horse and the cart in forward direction.

Mention the condition when an object in motion
a) can be considered point object
b) can not be considered point object

a) it can be considered as a point object. When the distance travelled by it is very large compared to the object. eg travelling from delhi to agra by car, here car is the object.

b)When distance travelled is comparable with the size of object.

Weight of an object is the _ force of the acting on an object.

The normal force acting on an object is equal to its weight.

So, the weight of an object is the normal force of the surface acting on the object.

A man pushes a wall with force $F = 10\ N$ towards the east. What force is exerted by the wall on the man?

Given,

Force applied by the man,

$F=10\ N$ ( towards east)

According to Newton's third law of motion, for every action there is an equal and opposite reaction.

Force exerted by the wall,

$F=10\ N$ (west)

What happens to the passengers travelling in a bus when the bus takes a sharp turn? Give reason for your answer.

Initially, the passengers and the bus both are moving in same direction. When the bus suddenly takes a sharp turn, the lower portion of passengers which is contact with the bus tends to move in that direction but the upper portion of the passengers throw an outside. This happens due to the INERTIA OF DIRECTION.

What type of forces-balanced or unbalanced-act on a rubber ball when we press it between our hands? What effect is produced in the ball?

When we press a rubber ball between our hands, it does not move or accelerate but the shape of the ball get changes. So according to newton's law; balance forces would be acting on the ball. Balance force can be able to change the shape and size of soft body.

Two bodies A and B of same mass are moving with velocities v and 2v respectively. Compare their
inertia.

Mass is the measure of inertia.

If 'm' is the mass of two bodies

$A$ and

$B,$
Comparing inertia of both the bodies;
Inertia of

$A:$ Inertia of

$B :: m : m \,or\, 1 : 1$

A body of mass m moving with a velocity v is acted upon by a force. Write expression for change in momentum when $v << c$ but m does not remain constant. Here $c$ is the speed of light.

When

$v << c;$

$\Delta p=\Delta mv$ .

Two equal and opposite forces act on a stationary body. Will the body move? Give reason to your answer.

When two equal and opposite forces are acting on a stationary body, the body will not move as the net force on the body is zero, hence the body will remain stationary. It will be in a state of rest due to inertia.

Why does a person fall when he jumps out from a moving train?

It is because inside the train, his whole body was in a state of motion will the train. On jumping out of the moving train, as soon as his feet touch the ground, the lower part of his body comes to rest, while the upper part still remains in motion due to inertia of motion.

Name the two factors on which the force needed to stop a moving body in a given time, depends.

Velocity and mass are the two factors on which the force needed to stop a moving body is dependent on a given time.

State Newton's second law of motion. What information do you get from it?

Newton's second law of motion states that:
The rate of change of momentum of a body is directly proportional to the force applied on it and the change in momentum takes place in the direction in which the force is applied Newton's second law of motion provided the quantitative value of force. We could relate the physical quantity force to the other measurable quantities such as mass and acceleration.

"The action and reaction both act simultaneously." Is this statement true?

Action and reaction both act simultaneously on different objects in opposite directions.
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If a man jumps out from a boat, the boat moves backward. Explain.

According to Newton's third law of motion, for every action, there is an equal and opposite reaction. When the man jumps from a boat, he applies force on the boat due to which boat moves backward. An equal force is exerted by the boat on the man which helps the man to jump out of the boat.
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Greater the mass, greater is the inertia of the body. Type 1 for true and 0 for false

1. Suppose two buckets are hanging above the ground. One bucket is filled with sand and other is empty. Initially both buckets are at the state of rest. Now if we want to push these buckets into a swinging motion, the bucket with sand would require more force, because it has more mass or inertia.
2. It is always difficult to pull out a truck from the mud than a car, only because a truck has more mass than a car.

'More the mass, the more difficult it is to move the body from rest'. Type 1 for true and 0 for false

Suppose two buckets are hanging above the ground. One bucket is filled with sand and other is empty. Initially both buckets are at the state of rest. Now if we want to push these buckets into a swinging motion, the bucket with sand would require more force, because it has more mass or inertia.

Comment on the statement 'the sum of action and reaction on a body is zero'.

The statement is wrong because Newton's third law of motion, that action is equal and opposite to the reaction, is always considered an interaction between two bodies and not one body. Sum of action and reaction forces on two interacting bodies is zero.

Can you identify the action and reaction forces in the case of an object falling under gravity? Neglect air resistance

The earth pulls a mass with a force of mass times gravity, the same force is experienced by earth as well, however, the mass of earth is too large to have any significant acceleration. This two force are action and reaction pairs.

To cause acceleration in an object the necessary factor is unbalanced _____________ .

Force,

According to Newton's second law,

$F_{net} = ma$

$F_{net}$ is non-zero when unbalanced forces acting on an object.

So, for acceleration, we need unbalanced Force.

If two horizontal forces $F_1$ and $F_2$ of the unequal magnitude act on a body in opposite directions then find the net force acting on the body.

The situation is shown in the above diagram. Here we have considered

$F_1 > F_2$ .

$F_1$ acts towards right and

$F_2$ acts towards left.

The net force acting on the body is

$F_{net} = F_1 - F_2$ (towards right).

Distinguish between balanced and unbalanced forces.

Balanced forces	Unbalanced forces
If a set of forces acting on a body produce no acceleration in it, then forces are said to be balanced.	If a set of forces acting on a body produces some non zero acceleration, forces are said to be unbalanced.
e.g. When we try to push a heavy box and it does not move, the force applied by us is balanced by the friction force acting on the box by the ground.	e.g. When we drag a box on the floor, there is an unbalanced force acting on the box.

Define spring balance.

A spring balance is a weighing apparatus used in industries to measure the mass of different loads. It is a type of weighing scale. It consists of a spring fixed at one end with a hook to attach an object at the other. It works by Hooke's Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position.

Correct the mistakes, if any, in the following statements.
i) One newton is the force that produces an acceleration of $1\ ms^{-2}$ in an object of $1$ gram mass.
ii) Action and reaction always act on the same body.

i) One Newton is the force that produces an acceleration of

$1\ ms^{-2}$ in an object of

$1\ Kg$ mass.

According to newton's second law-

$F = ma$

$\Rightarrow m = \dfrac{ F }{ a }$

$\Rightarrow m = \dfrac{ 1 }{ 1 }$

$\Rightarrow m = 1\ kg$

ii) Action and reaction always act on two different bodies.

Why does a fielder in the game of cricket pull his hands back when he catches a ball?

According to Newton's second law, rate of change of momentum is directly proportional to the force applied. When a fielder catches a ball, the momentum of the ball becomes zero.

Now, rate of change of momentum =

$\dfrac{\bigtriangleup p}{\bigtriangleup t}$

To minimize the rate of change of momentum,

$\bigtriangleup t$ must increase, therefore by pulling his hands back, the fielder is increasing the time in which the momentum becomes zero, and thereby reducing the force exerted on his hands.

What happens when you shake a wet piece of cloth ? Explain.

When we shake a wet cloth, water droplets come off from it, because of inertia of rest. Water drops in the wet cloth were in a position of rest. When shaken, the cloth moves suddenly, but the droplets have a tendency to remain at rest owing to the property of inertia and they are detached from the cloth.

Explain rocket propulsion.

When rockets leave the launchpad, they need a lot of power to lift them off the ground. Accomplishing this isn't easy because the rocket can weigh up to

$4.4$ million pounds and hence due to this, rockets have to rely on a powerful propulsion system. Rocket propulsion is the process that uses a force to move a rocket off the ground and into the atmosphere.

There are two types of rocket propulsion required to launch the rocket off the ground, namely:

(1)

$Solid \ propulsion$ : Where the fuel and the oxidizer are combined as solid.

(2)

$Liquid \ propulsion$ : Where the fuel and the oxidizer are stored separately and then later combined and ignited.

By a bullet fire by a gun is more dangerous than a bullet fire with medium speed?

According to Newton's third law, when we fire a bullet we get a recoil of the gun due to action-reaction.

And, the momentum of recoil = the momentum of fired bullet

$= mass \times velocity = mv$

So, from here it is quite clear that the velocity of the fired bullet more will be the momentum results in more dangerous recoil.

$\text{Force} = \dfrac{ \text{Change in momentum} }{ \text{time} }$

If change in the momentum is large then value of force will be large.

Therefore, bullet fire by a gun is more dangerous than bullet fire with medium speed.

The mass of a cannon is $500\ kg$ and it recoils with a speed of $0.25\ m/s$ . What is the momentum of the cannon?

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Guess and write, a heavy truck causes more damage to the wall than a bicycle if both hit a wall with same speed. Why?

The amount of damage is determined by the amount of force applied on the wall.

A heavy truck in motion has very high momentum compared to a moving bicycle as its mass is thousands of times more then a bicycle.Since force is proportional to the rate of change of momentum, the truck will apply a far greater force on the wall than the bicycle. That's why a truck will do more damage to the wall compared to a bicycle.

Which would require a greater force-accelerating a $1$ kg mass at $10$ $m/s^{-2}$ or a $2$ kg mass at $4 m/s^2$ .

By second law of motion we have the formula as

Force = mass $\times$ acceleration

Given Force $1 = 1 \times 10 = 10$ Newton

Given Force $2 = 2 \times 4 = 8$ Newton

Hence, force $1$ is greater than force $2$ .

SI unit of force is Newton. $1 kg = nearly\ 9.8 N$

A person standing in a moving bus falls backwards when bus suddenly starts moving in forward direction. This happens due to

A person standing in a bus falls backwards when bus suddenly starts moving in forward direction. This happens because when bus starts moving forward, lower part of the person begin to move with the bus whereas the upper part of the body tends to remain in rest due to inertia.

State the relation between force and momentum.

According to newton's second law force is related to momentum in such a way that the force acting on a body is equal to the rate of change of momentum of the body.It is given as:

$F=\dfrac{\Delta{P}}{\Delta{t}}$

When a Carpet is beaten with a stick, dust comes out of it. Explain.

When a carpet is beaten with a stick, the carpet is set into motion. Due to inertia of rest, the dust particles tend to remain at rest . As a result, the dust particles fall off.

................ is a measure of the inertia of a body.

Mass is the measure of inertia of a body.

If action is always equal to reaction;explain

According to Newton's third law of motion, action force is equal to reaction but acts on two different bodies and in opposite directions. When a horse pushes the ground, the ground reacts and exerts a force on the horse in the forward direction. This force is able to overcome friction force of the cart and it moves.

Fill in the blanks:
A moving football when kicked, its _________ changes.

A moving football when kicked, its Direction of motion changes.

Name the force which is responsible for changes in position or state of an object.

$\textbf{According to the Newton’s First Law Of Motion}$

Every body remains in its state of rest or uniform motion in a straight line unless it is compelled by some external force.

It means a body remains uncelebrated if and only if, the resultant force on it is zero.

Hence, It is the $\textbf{Unbalanced external force,}$ which causes changes in the state of rest or motion.

$\textbf{Example of First Law Of Motion}$

SCIENCE FACTS: Reason of dropping a coin straight into the glass by removing a card suddenly?

$\bullet$ According to Newton's law of motion, If no external force is applied on a body, a stationary body will remain stationary and a moving body will continue to move without changing its speed.

$\bullet$ A body continues to do whatever it happens to be doing unless a force is exerted upon it. In the absence of net forces, a moving body tends to move along a straight-line path indefinitely.

$\bullet$ It is the property of the body to resist changes in motion that is widely known as inertia.

$\bullet$ To show the property of that body experimentally, a card is put on the top of a glass with a coin on the upper face of the card. Now If we accelerate the card with a force, the coin will drop straight into the glass.

$\bullet$ As we said earlier, a stationary body continues to remain stationary due to inertia. As the coin was initially in stationary state, after removing the card very rapidly, it wanted to continue its state of rest. That is why it dropped straight into the glass.

If the action is the cork being pushed out to the force exerted by the steam on it, what is the reaction?

The reaction is the backward movement of boiling tube.

How much force is needed to increase velocity of a body of $4kg$ mass by $1.5m/s$ every second?

Given, mass

$=4kg$ .
Acceleration = Rate of change of velocity

$=1.5m/s$ every second

$=1.5m/s^{2}$

$\therefore Force=mass\times acceleration$ ,

or,

$Force=4\times 1.5=6N$ .

Force produced by shooting makes the bullet to move forwards, also equal force in opposite direction is applied which recoils gun backwards. It is used to refill gun in semi automatic weapons.
State True or False.

It is due to third law of motion. According to it, force produced by shooting makes the bullet to move, also applies equal force in opposite direction which recoils the gun backwards. It is used to refill gun in semi automatic weapons. When a bullet is fired from a gun, the gun exerts a force on the bullet in the forward direction. This is the action force. The bullet also exerts an equal force on the gun in the backward direction. This is the reaction force. Due to the large mass of the gun it moves only a little distance backward by giving a jerk at the shoulder of the gunman. The backward movement of the gun is called the recoil of the gun.

Give two examples each of situation in which you push or pull to change the state of motion of an object.

Below are the examples of situations where we push or pull to change the state of motion of an objects.

We have to push a lawn mower.
We have to push a punctured bike on road.
We pull the table to change its position.
To draw water from a well we have to pull the rope.

A pebble of mass $0.05 kg$ is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble,
(a) during its upward motion,
(b) during its downward motion,
(c) at the highest point where it is momentarily at rest. Do your answers change if the pebble was thrown at an angle of $45^o$ with the horizontal direction?
Ignore air resistance.

$0.5 N$ , in vertically downward direction, in all cases
Acceleration due to gravity, irrespective of the direction of motion of an object, always acts downward. The gravitational force is the only force that acts on the pebble in all three cases. Its magnitude is given by Newtons second law of motion as:

$F=ma$
Where,

$F =$ Net force

$m =$ Mass of the pebble

$= 0.05 kg$

$a = g =$

$10 m/s^2$

$F = 0.05$

$\times$

$10 = 0.5 N$
The net force on the pebble in all three cases is

$0.5 N$ and this force acts in the downward direction.

If the pebble is thrown at an angle of

$45^o$ with the horizontal direction, it will have both the horizontal and vertical components of velocity. At the highest point, only the vertical component of velocity becomes zero. However, the pebble will have the horizontal component of velocity throughout its motion. This component of velocity produces no effect on the net force acting on the pebble.

According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.

Hint: Two equal and opposite forces cancel each other if they applied on the same body.

Solution:

Step1: The justification given by the student is wrong. Because the forces of action and reaction do not act on the same body (they act on two different bodies here). So, there are no chances of their cancellation.

Step2: The massive truck does not move in such case because a force applied to it is much smaller than the force of friction between the wheels of the truck and the road. Since the force of push is not able to overcome the force of friction. Therefore, the truck does not move.

Why is it advised to tie any luggage kept on the roof of a bus with a rope?

When the bus moves in forward direction, the luggage kept on the roof also moves along with the bus in forward direction. Now when the bus suddenly stops, it comes to rest but the luggage is still in motion in forward direction due to inertia of motion, thus the luggage falls off the roof of bus.

Hence it is advised to tie it.

Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar(because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.

Kiran's statement:

Change in momentum of the insect and the motorcar is equal by conservation of momentum. The velocity of insect changes significantly because its mass is very small compared to the motorcar. Similarly, the velocity of motorcar is very insignificant because its mass is very large compared to the insect. Hence, Kiran's statement is false.

Akhtar's statement:

By newton's third law of motion, action and reaction force are equal and opposite and they act on different bodies. Hence, the motorcar and insect both experience the same force. Since the mass of the insect is very small compared to the motorcar, it suffers a huge change of velocity as compared to the car. Due to this, insect dies. Hence, Akhtar's statement is false.

Rahul's Statement:

As discussed before, both the motorcar and the insect experience the same force and change in momentum. Hence, Rahul's statement is correct.

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

$\textbf {Hint :}$ When equal and opposite forces act on any object, then they cancel out each other so that the total external force experienced by the object is zero.

$\textbf{Step1: Draw free body diagram}$

$\textbf{Step2: Calculate net force}$

So net force, $F=m\times a$

Where, F=Net force, m=Mass, a=Acceleration.

Hence, If net force (F) =0, then acceleration (a) will also be equals to zero (because mass ‘m’ cannot be zero).

Therefore, acceleration in an object could be zero even when several forces are acting on it. Hence, Agree with Soni.

Give reasons for the following;
An athlete in the long jump runs a distance before taking the leap.

An athlete runs a distance before taking the leap so as to build his momentum so that he can jump a longer distance.

As athlete starts running, his velocity starts increasing and as momentum is product of mass and velocity, it also starts increasing. When he finally makes the jump, he can cover a longer distance due to larger momentum.

Explain why some of the leaves may get detached from a tree if we vigorously shake it branch.

The answer of this cause lies behind the Newton’s First Law of Motion. Initially, leaves and tree both are in rest. But when the tree is shaken vigorously, tree comes in motion while leaves have tendency to be in rest. Thus, because of remaining in the position of rest some of the leaves may get detached from a tree if we vigorously shake its branch.

Give reasons for the following
Usually a fielder in a cricket match, takes a catch by moving his hands in the direction of motion of cricket ball.

While catching a fast-moving cricket ball, a fielder in the ground gradually pulls his hands backward with the moving ball. In doing so, the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the acceleration of the ball is decreased and therefore the impact of catching the fast-moving ball is also reduced. If the ball is stopped suddenly then its high velocity decreases to zero in a very short interval of time. Thus, the rate of change of momentum of the ball will be large. Therefore, a large force would have to be applied for holding the catch that may hurt the palm of the fielder.

State any two illustrations for Newtons third law of motion.

1. While walking on a road, the person pushes the road backward. The road exerts an equal and opposite reaction force on the feet of the person to make him move forward.

2. When a gun is fired, it exerts a force on the bullet, whereas the bullet exerts an equal and opposite force on the gun, consequently the gun recoils.

From the following statements write down which is not applicable to mass of an object.
(a) It is a fundamental quantity.
(b) It is measured using physical balance.
(c) It is measured using spring balance.

Spring balance is a device used to measure the weight of the object.

Weight is defined as the force experienced by the object due to earth's gravity. Mass is defined as the total amount of matter contained in an object.

Why does a boxer always move along the direction of the punch of the opponent?

As according to Newton second law of motion. Change in momentum per unit time is equal to the force.

$\vec{F}=\frac{-\triangle p}{\triangle t}$

So increase the momentun of its punch so as to increase. The force exerted the Boxer move along the direction of punch.

Write the principles that are used in rocket propulsion.

The propulsion of all rockets is explained by the same physical principle: Newton's third law of motion. A rocket's acceleration depends on three major factors: the exhaust velocity, the rate the exhaust is ejected, and the mass of the rocket.

A cart driver prods his horse to move forward. The horse refuses to budge and explains:
According to Newton's III Law, I am pulling the cart, with a certain force and the cart, in turn, pulls me back with an equal amount of force. As they are equal in magnitude and act in opposite directions, they cancel each other.
Do you agree with the explanation given by the horse? Support your answer with proper reasons.

Yes, the horse explanation is right that cart and horse form action and reaction pair but there is also one another force which causes the whole system to move which is the muscular force by horse supported by the friction.

The mats used in gyms and the padding used in sports uniforms are made up of soft substances. Why are rigid materials not used?

Because the soft substances move towards the direction of motion as they are less elastic

$F_{exerted}=\frac{\triangle p}{\triangle t}$ (Newton second law)

Because of the softness, the substance undergoes less change in momentum (

$\triangle p$ is small).

So because of small

$\triangle p$ , the force exerted is small.

Consider a head-on collision between two particles of masses $m_1$ and $m_2$ . The initial speeds of the particles are $u_1$ and $u_2$ in the same direction. The collision starts at t = 0 and the particles interact for a time interval $\Delta t$ . During the collision, the speed of the first particle varies as
$v(t) = u_{1} + \frac{t}{\Delta t}(v_{1}- u_{1})$
Find the speed of the second particle as a function of time during the collision.

Given mass of particle

$1={ m }_{ 1 };\quad mass\quad of\quad particle\quad 2=\quad { m }_{ 2 }$

Using law of conservation of momentum,

${ m }_{ 1 }{ u }_{ 1 }+{ m }_{ 2 }{ u }_{ 2 }={ m }_{ 1 }v(+)+{ m }_{ 2 }V$

(

$({ v }^{ 1 }=\quad Speed\quad of\quad second\quad particle\quad during\quad Collision)\\$

${ V }^{ 1 }=(\dfrac { { m }_{ 1 } }{ { m }_{ 2 } } ){ V }_{ 1 }+{ V }_{ 2 }-(\dfrac { { m }_{ 1 } }{ { m }_{ 2 } } )V(t)\\ { V }^{ 1 }=(\dfrac { { m }_{ 1 } }{ { m }_{ 2 } } ){ V }_{ 1 }+{ V }_{ 2 }-(\dfrac { { m }_{ 1 } }{ { m }_{ 2 } } ).({ U }_{ 1 }+(\dfrac { t }{ \Delta t } )({ V }_{ 1 }-{ V }_{ 1 })\}$

${ V }^{ 1 }={ U }_{ 2 }-(\dfrac { { m }_{ 1 } }{ { m }_{ 2 } } ).(\dfrac { t }{ \Delta t } )({ V }_{ 1 }-{ U }_{ 1 })$

Two equal forces to P and 2P act on a particle.If the first be doubled and the second be increased by 20N, the direction of the resultant is unaltered.Find the value of P.

Let the resultant make angle

$\beta$ with the force P.

$In\quad first\quad case\quad ,\quad \quad \quad \quad \quad \quad \quad \tan { \beta =\frac { 2P\sin { \theta } }{ P+2P\cos { \theta } } } \\ In\quad second\quad case,\quad \quad \quad \tan { \beta \quad =\quad \frac { \left( 2P+20 \right) \sin { \theta } }{ 2P+\left( 2P+20 \right) \cos { \theta } } } \\ \frac { \left( 2P+20 \right) \sin { \theta } }{ 2P+\left( 2P+20 \right) \cos { \theta } } \quad =\frac { 2P\sin { \theta } }{ P+2P\cos { \theta } } \\ \left[ \because \frac { a }{ b } =\frac { c }{ d } =\frac { a-c }{ b-d } \right] \\ we\quad get\quad \\ 2P\quad =\quad 20\\ P=10\quad N\\$

Which would require a greater force?
Accelerating a $5\ kg$ object at $3\ m/s^2$ , or an $8\ kg$ object at $2\ m/s^2$ ?

1379407_1006030_ans_d1a8c496cbb24d1d9681b69beeeda31e.jpg

Find F for body to be in equilibrium.

For equilibrium, net force acting on the body will be zero

$\sum { { F }_{ x } } =0\\ \therefore 10-F=0\\ \Rightarrow F=10N$

Action and reaction are equal and opposite. Why cannot they cancel each other?

Newton's Third Law really does say that if A pushes B, then B pushes A with an equal and opposite force. However, these forces DO NOT CANCEL because they influence the motion of different objects. The force that A exerts on B influences B's motion, and the force that B exerts on A influences A's motion. The force on B can cancel with other forces on B - but NOT with forces on A(and vice versa).

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Explain Newton's second law of motion with the help of an example?

According to Newton's second law of motion, the rate of change of a body/object is directed proportional to force applied to it and takes place in the direction in which the force acts.

$Force= Mass \times Acceleration$

You may have noticed that while catching the faster-moving cricket ball,a fielder often pulls his hand backward with the moving ball. This is because In doing so, the fielder increases the time during which the high velocity if the moving ball decreases to zero. Therefore, the Acceleration of the ball decreases and therefore impact of catching the faster-moving ball is also reduced and the fielder did not get hurt in his hand.

A force of $12\ N$ pulls a block of mass $3.5\ kg$ through a rope of mass $0.5\ kg$ . The block is resting on a horizontal frictionless surface. What is the force of reaction exerted by the block on the rope?

Given,

Force applied is $12\,N$

$a=\frac{F}{ m_{total}}$ = $\frac{12}{3.5+.5}$ = $\frac{12}{4}$ = $3m/s^2$

$Tension=m_{block}\times{a}$ = $3.5\times{3}$ = $10.5N$

Reaction force = Tension applied on block (In Magnitude)

Reaction force exerted by block is $10\,N$ .

A hockey ball of mass 200g is traveling at 10 m/s and is struck by a hockey stick, so as to return it along its original path with a velocity of 50 m/s. Calculate the change of momentum occurred in the motion of hockey ball by the force applied by the hockey stick.

$\underline{\text{Initial Situation:}}$

Mass of the hockey ball

$m=200\ g = 0.2\ kg$

Velocity of the ball

$v_1=10\ m/s$

Thus, initial momentum of the ball is

$P_1=mv_1=0.2\times 10 = 2\ kg-m/s$

$\underline{\text{Final Situation:}}$

Mass of the hockey ball

$m=200\ g = 0.2\ kg$

Velocity of the ball

$v_2=50\ m/s$

Thus, final momentum of the ball is

$P_2=mv_2=0.2\times 50 = 10\ kg-m/s$

The hockey ball is returned to its initial path after being hit by the stick. Therefore, the direction of the initial momentum and the final momentum is opposite.

$\therefore \text{The change in the momentum}=\Delta P=P_2-(-P_1)=P_2+P_1=2+10 = 12\ kg-m/s$

Guess and write why does pole vault jumper land on thick mats of foam?

In a pole vault jump event, the athletes are made to fall on thick mats of foam. This is to increase the time of the athlete’s fall to stop after making the jump. This decreases the rate of change of momentum and hence the force. If there was no mat, the athlete's body on contacting with the ground would instantaneously come to rest, thus resulting in a large force and them hurting themselves. It can be seen mathematically as well,

$F=\dfrac{\Delta P}{\Delta t} \Rightarrow F \propto \dfrac{1}{\Delta t}$

Time interval

$\Delta t \uparrow \ \Rightarrow$ Force

$F \downarrow$

Write and explain Newton's second law of motion.

Newton's second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object

(a). Name the forces acting on a plastic bucket containing water held above ground level in your hand.

(b). Discuss why the force acting on the bucket do not bring change its state of motion ?

(c). Why do porters place a round of cloth on their heads when they have to carry heavy loads?

(a). The forces acting on a plastic bucket containing water held above ground level in your hand are- 1)gravitational force 2)muscular force

(b). The forces acting on the bucket do not bring a change in its state of motion because both forces acting opposite directions are equal and hence, net force on the bucket is zero.

(c). Head surface shape do not match completely with the surface in contact of load due to which pressure distribution occur randomly,

Some portion of head get highly stressed wheres some portion left untouched.

Cloth change its shape to such extent according to load apply on it.

Hence, by wearing cloth on head, pressure gets distribute more evenly on all the surface and porters feels easy to carry the load.

A 10 g bullet is shot from a 5 kg gun with a velocity of 400 $m{s^{ - 1}}$ .What is the speed of recoil of the gun?

Given,

Mass of the gun, $M =5kg$

Initial Velocity of gun, $U_1=0 m/s$

Mass of the bullet $, m = 10 gram = 0.010 kg$

Initial Velocity of Bullet(Before fire), $U_2=0 m/s$

Let final Recoil velocity of Gun, $=V_1$

Final velocity of bullet,(After fired) , $V_2 =400m/s$

By conservation of momentum

${m_1}{U_1} + {m_2}{U_2} = {m_1}{V_1} + {m_2}{V_2}$

Or $M \times 0 + m \times 0 = M{V_1} + m \times 400$

Recoil speed of the gun, ${V_1} = \dfrac{{ - 400m}}{M} = \dfrac{{ - 400 \times 0.01}}{5} = - 0.8m/s$

A person getting out of a moving bus on a train falls in froward direction.

This is the same reason that the same person who is standing leans toward when a bus stops,

leans left or right when a bus turns Orleans back when a bus accelerates. This is all due to inertia.

You are moving you are going to go the same direction as you were in the bus.

Basically the law of inertia states when an object is at rest stays at rest

and an object in motion stays in motion until an outside force is acted upon that object.

Because of this you are moving with the bus until your feet touch the ground and the ground

along with your muscles prevent you from moving.

An object of mass $10\ kg$ is moving with an initial velocity of $2\ ms^{-1}$ . To make it come to rest in one second, what should be the unbalanced force?

Initial velocity

$u= 2 \,ms^{-1}$
velocity at rest

$v = 0\,ms^{-1}$
time interval

$t = 1$ second
mass

$m = 10\,kg$
change in velocity in one second or acceleration

$a = \dfrac{\text{Final velocity - Initial velocity}}{t}$

$a= \dfrac{0 - 2}{1}$

$\Rightarrow a = 2 \, ms^{-1}$

Apply newton's second law

$F = ma$

$F = 10 \times (-2)$

$20\,kg\,ms^{-2}$ or

$-20$ newton or

$-20\,N$ . The negative sign tells us that the force exerted is opposite to the direction of motion.

Why is it difficult to walk on a slippery road?

It becomes very difficult to walk on slippery road because of the fact that on a slippery road, the friction is much less and we cannot exert a backward action force on slippery ground which would produce a forward reaction force on us.

If a balloon filled with air and its mouth untied, is released with is mouth in a downward direction,it moves upwards. Why?

If a balloon filled with compressed air and its mouth untied is released with its mouth in the downward direction, the balloon moves in the upward direction because the air present in the balloon rushes out in the downward direction.The equal and opposite reaction of downward going air pushes the balloon upwards.

What is the effect of unbalanced forces?

The effect of force will be observed when an unbalanced force acts on an object.
Unbalanced forces acting on an object change its speed or direction of motion.
1772766_1452150_ans_94dcc35715c2472e94da8a9bd0c39fe5.png

1772766_1452150_ans_94dcc35715c2472e94da8a9bd0c39fe5.png

Explain why, a runner presses the ground with his feet before he starts his run.

To start his run,a runner bends forward and pushes the ground with his feet in the backward direction.In turn ground exerts a reaction force on the runner in the forward direction which makes him run.

What is the total momentum of the bullet and the gun before firing?

The total momentum of the bullet and the gun before firing would be zero because velocities of both of them will be zero.

What is the usual name of the forces which can not produce motion in a body but only change its shape?

Balanced forces cannot produce motion in a body but can change its shape.

Unbalanced force creates internal tension in the object hence unbalanced force changes the shape of the object.

A fat person is standing on a light plank floating on a calm lake. The person walks from one end to the other on the plank. His friend sitting on the shore watches him and finds that the person hardly moves any distance because the plank moves backward about the same distance as the person moves on the plank. Explain.

Since the plank is light its mass is negligible in comparison to the fat person. Combined center of mass of the system containing the fat person and the plank can be considered near the person. Since there is no external force on this system the center of mass of the system will remain at the same place. Due to this the friend on the shore finds the fat person hardly move.

Which of the two,balanced forces or unbalanced forces,can change the shape of an object ? Give an example to illustrate your answer.

Balanced forces can change the shape of the object.For example: when a balloon is pressed between hands, then balanced forces (equal and opposite forces) act on the balloon due to which the shape of the balloon changes.

When a stationary car starts suddenly, the passengers are jerked ________.

When a stationary car starts suddenly, the passengers are jerked in the backward direction. Because when the car starts and moves suddenly in the forward direction from a state of of rest, the passengers experience a force exerted on them in the backward direction due to the inertia opposing the forward motion.

Name the property of bodies (or objects) to resist a change in their state of rest or of motion.

Inertia is the property of bodies to resist a change in their state of rest or motion.

Name the force which slows down a moving bicycle when we stop pedalling it.

The frictional force, acting between the road and the cycle, slows down a moving bicycle when we stop pedaling it.

The mass of object A is 6 kg whereas that of another object B is 34 kg. Which of the two objects A or B, has more inertia?

Object B has more inertia.Since mass is a measure of inertia of a body and object B has greater mass,so it will have greater inertia.

When a tree is shaken, its fruits and leaves fall down: why?

When a tree is shaken,the tree moves to and fro while the fruits and leaves remain in their state of rest on account of their inertia and thus fruits and leaves separate from the tree and fall from the tree.

Why do the passengers in a bus tend to fall backward when it is starts suddenly?

When a bus starts suddenly,its passengers tend to fall backwards because due to their inertia, the passengers tend to remain in a state of rest even when the bus starts moving.

What do you understand by the terms "balanced forces" and "unbalanced forces"? Explain with examples.

If the resultant of all the forces acting on a body is zero, then forces are called balanced forces. These forces do not change the state of rest or of uniform motion of body but can change the shape of the body.

Example: When we push a concrete wall with our hands the wall doesn't move. This is because the force by us is balanced by the other forces acting on the wall.
If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces. These forces change the state of rest or of uniform motion of a body.

Example: If we push a toy car lying on the ground, it starts moving due to the unbalanced force exerted by our hands.

The velocity of a body of mass 10 kg increases from 4 m/s to 8 m/s when a force acts on it for 2 s.
a) what is the momentum before the force acts?
b) What is the momentum after the force acts?
c) what is the gain momentum per second?
d) What is the value of the force?

Mass of the body

$=10$ kg
Initial velocity

$u=4$ m/s
Final velocity

$v=8$ m/s
Time

$t=2$ s
a) Momentum before force acts

$p_1=m \times u=10 \times 4=40 kg.m/s$
b) Momentum after force acts

$p_2=m \times v=10 \times 8=80 kg.m/s$
c) Gain in momentum per second

$=\dfrac{40}{2}=20 kg.m/s$
d) Acceleration

$a=\dfrac{v-u}{t}=\dfrac{8-4}{2}=2 m/s^2$
Force

$= m \times a =10 \times 2 =20$ N

Which physical principle is involved in the working of a jet aeroplane?

The physical principle involved in the working of a jet aeroplane is Newton's third law of motion. According to Newton's third law of motion, for every action there is an equal and opposite reaction. Jet aeroplane exerts a backward force on air and air exerts an equal and opposite reaction force on the jet aeroplane.

What is the relationship between force and acceleration?

Acceleration of a body is is equal to the ration of applied force to mass.

$a=\dfrac FM$

Do action and reaction act on the same body or different bodies? How are they related in magnitude and direction? Are they simultaneous or not?

Action and reaction act on two different bodies.Action and reaction are equal in magnitude but they act in opposite directions and there is simultaneous action and reaction.

Why does a gunman get a jerk on firing a bullet?

Gunman gets a jerk on firing a bullet because when a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backwards but due to high mass of the gun, it moves only a little distance backwards giving a jerk to the gunman.

A 60 g bullet fired from a 5 kg gun leaves with a speed of 500 m/s.Find the speed (velocity) with which the gun recoils (jerks backwards).

Given:

Mass of bullet,

$m_1=60g=0.06 kg$
Velocity of bullet

$v_1=500 m/s$
Mass of gun

$m_2=5 kg$
Recoil velocity

$v_2$
According to the law of conservation of momentum

$m_1 \times v_1=m_2\times v_2$

$0.06\times 500 =5\times v_2$

$v_2=\dfrac{0.06 \times 500}{5}=6 m/s$

A gun of mass 3 kg fires a bullet of mass 30 g.The bullet takes 0.003 s to move through the barrel of the gun and acquires a velocity of 100 m/s.Calculate:
i) the velocity with which the gun recoils.
ii) the force exerted on gunman due to recoil of the gun.

Mass of the gun

$m_1=3 kg$
Mass of the bullet

$m_2$ =30 g=0.03 kg
Velocity of bullet

$v_2$ =100 m/s
i) According to the law of conservation momentum

$m_1 \times v_1=m_2\times v_2$

$3 \times v_1=0.03 \times 100$
Recoil velocity

$v_1=\dfrac{100 \times 0.03 }{3}=1 m/s$
ii) Initial velocity of the gun u=0 m/s
Final velocity of the gun v=1m/s
Time t=0.003 s
Acceleration

$a=\dfrac{v-u}{t}=\dfrac{1-0}{0.003}=\dfrac{1000}{3} m/s^2$
Force=

$m \times a =3 \times \dfrac{1000}{3}=1000 N$

Why is it advised to tie the luggage with a rope on the roof of buses?

Luggage in contact with the roof of the bus. When the bus is moving, luggage is also in the state of motion and had a tendency to remain in motion due to the law of inertia. When the driver applies the brakes, the bus stops suddenly but the luggage is in motion and thus falls from the roof. It is therefore advised to tie the luggage.

A man wearing a bullet-proof vest stands still on roller skates.The total mass is 80 kg.A bullet of mass 20 grams is fired at 400 m/s.It is stopped by the vest and falls to the ground.What is then the velocity of the man?

Given:

Mass of the man,

$m_1=80 kg$
Speed of man after collision,

$v_1$
Mass of bullet

$m_2=20 g=0.02 kg$
Speed of bullet

$v_2=400 m/s$
Acc to law of conservation of momentum

$m_1v_1=m_2v_2$

$80 \times v_1=0.02 \times 400$

$v_1=\dfrac{400 \times 0.02}{80}=0.1 m/s$

When a carpet is beaten with a stick, dust comes out, why?

According to Newton's law, an object which is at rest or in motion will continue its state of rest or motion unless and until an external force is applied to it. This property is known as inertia. Dust particles are at rest with respect to the carpet. When the carpet is beaten, the dust due to inertia tends to remain at rest while the carpet moves due to an external force applied. So, the dust comes out of the object.

Difference between First and second law of motion.

First Law of Motion	Second law of motion
1. Newton's first law states that an object at rest will stay at rest and an object in motion will stay in motion with the same speed and direction unless and until an external force is applied on the object.	1. Newton's second law states that the rate of change of momentum of a body is directly proportional to the force applied, and this change in momentum takes place in the direction of the applied force.
2. This law is also known as law of inertia.	2. This law is also known as law of force and acceleration.
3. For example: when brakes are applied we tend to move forward, car takes sharp turn then we are thrown outwards.	3. For example: when we push a small stone and a huge rock then the stone will be accelerated because it has less mass.

Explain why it is possible for a small animal to fall from a considerable height without any injury being caused when it reaches the ground.

It is possible for a small animal to fall from a considerable height without being injured because a small animal has small mass,so the momentum produced is less.When the small animal falls to the ground with less momentum, less opposing force of ground acts on it and hence no injury is caused to it.

Two balls of the same size but of different material, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed?
Give reason for your answer.

Yes, the balls will starts rolling in the direction in which the train was moving. Due to the application of the brakes, the train comes to rest but due to inertia the balls try to remain in motion, therefore they begin to roll. Since the mass of the balls are not same, therefore , the inertia of iron ball is greater than rubber ball due to mass. Thus, the rubber ball will move fast.

A body at rest opposes the force which tries to move it. Name this property and give two examples to illustrate it.

This is an inherent property called inertia of rest which is directly proportional to the mass of the object. Examples:-
1. When we beat a carpet, dust particles come out.

2. When you shake the branch of a tree, fruits or leaves come out.

A gas-filled balloon moves up. Is the upward force acting on it larger or smaller than the force of gravity?

The upward force which is helping the balloon to move upward will be greater than the force of gravity.

Sonam was reading about balanced and unbalanced forces. She read that the gravitational force of the sun and moon also act on us. She wondered why we don't fly towards these bodies. Her teacher. however, listened to her patiently and stored her doubts.
(a) What values of Sonam do you observe?
(b) Throw light on the characteristic values of her teacher.
(c) Distinguish between balanced and unbalanced forces.
(d) What explanation would you give towards Sonam's questions?
(e) Good reading habits lead to constructive learning. Comment.

(a) We observe that Sonam is very curious about understanding natural phenomena about the gravitational forces between the sun and moon.

(b) The characteristic values of her teacher are as followed:

Expert communication skills, superior listening skills, deep knowledge and passion for their subject matter, the ability to build caring relationships with students, friendliness, and approachability, and excellent organization skills.

$F_{net} = 0 \,N$ , whereas Unbalanced forces are those in which

$F_{net} \neq 0 \,N$ .

(d) Gravitational force is a weak force, and it depends inversely upon the square of the distance.

$F_{grav} \propto \dfrac{1}{r^2}$ . The earth is much closer to us than either the sun or the moon, so the gravitational pull of the earth is more effective than the other two.

(e) Good reading habits like reading newspapers, science journals lead to learning of the new technology and ideas in one's dictionary, which allows him/her to question existing theories, which results in constructive learning.

A horse continues to apply force in order to move a cart with a constant speed. Explain.

A horse continues to apply force in order to move a cart with a constant speed. If the horse stops applying force, the cart begins to slow down. This is because of the friction forces acting opposite to the direction of motion. In order to keep the cart moving, the horse has to apply force again. The cart will maintain its motion under the continuous application of an

unbalanced.

The cart moves with a constant speed when the forces (pushing force and frictional force) acting on the object are balanced and there is no net external force on it.

Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.

The working of the rotation of is based on third law of motion. As the water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play. So, the sprinkler starts rotating.

Two identical bullets are fired one by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why ?

According to law conservation of momentum, the momentum of bullet forward will be equal to the momentum of rifle backward. In case of light rifle, velocity will be more than the velocity of heavier rifle because of mass so that momentum (product of mass and velocity) for both shall not be equal. Due to this, the lighter rifle will hurt the shoulder more. In both cases it has called recoied.

A ball moving on a surface stops after travelling some distance. Does it violate the first law of motion? Explain.

The first law of motion states that a body in motion stays in motion and a body at rest stays at rest until an external force is applied. But in the case of rolling of the ball, the ball stops after some time because the frictional force is acting on the ball every time which keeps on reducing its speed. Hence, it does not violate the first law of motion.

State Newton's second law of motion. Under what conditions does it take the form F=ma?

Newton's second law of motion - The rate of change of momentum of a body is directly proportional to the force applied on it and this change in momentum takes place in the direction of the applied force, i.e.

$F \propto \dfrac{\Delta p}{\Delta t}$

where p denotes momentum and

$\Delta t$ is the change in momentum in time

$\Delta t$ .

When mass m of the body is constant at velocity v (which is much smaller than the velocity of light c) then the rate of change of momentum is:

$F \propto \dfrac{\Delta p}{\Delta t}=m \dfrac{\Delta v}{\Delta t}=ma$

$F=ma$

What is meant by the term inertia?

Inertia is the tendency of an object to resist any change in its state of rest or motion.

'More the mass, more difficult it is to move the body from rest'. Explain this statement by giving an example.

Mass of a body is directly related to inertia, the more mass of an object, the more the inertia. Compared to an unloaded trolley, it is difficult to set a loaded trolley in motion as a larger force is required to set it to motion.

How can you feel a force of $1$ N?

From Newton's law:

$F=ma$

When

$m=1\ kg$ ,

$F=1\ N$ then

$a=\dfrac{F}{m}=1\ m/s^2$

One Newton is that force which, when acting on a body of mass

$1$ kg, produces an acceleration of

$1\,m s^{-2}$ in it.

A bullet of mass $50 \ g$ moving with an initial velocity of $100 \ m/s$ , strikes a wooden block and comes to rest penetrating a distance $2 \ cm$ in it. Calculate final momentum of the bullet.

Given:

Mass of the bullet

$m = 50\ g =0.05\ kg,$

Initial velocity

$u = 100 \ m/s,$

Final velocity

$v = 0 \ m/s,$

$s = 2\ cm=0.02 \ m$

Final momentum of the bullet

$P_f= mv$

$= 0.05 \times 0 = 0$

How does Newton's second law of motion differ from first law of motion?

The differences are as given above:
1706799_1809289_ans_0ca1ffae6a374feb96657f118abe501b.png

When a shot is fired from a gun, the gun gets recoiled. Expain.

When a man fires a bullet from a gun, a force F is exerted on the bullet (action), and the gun experiences an equal and opposite recoil (reaction) and hence gets recoiled.
1707900_1810198_ans_1cb31037ae9942dd8ba82d9f924631a3.png

1707900_1810198_ans_1cb31037ae9942dd8ba82d9f924631a3.png

Name and state the action and reaction in the following cases:
(a) Firing a bullet from a gun,
(b) Hammering a nall,
(c) A book lying on a table,
(d) A moving rocket,
(e) A person walking on the floor,
(f) A moving train colliding with a stationary train.

A) Action - Firing bullet

Reaction - Recoiling of gun

b) Action - Hitting hammer on nail

Reaction - the nail exerts equal force on

hammer.

c) Action - book exerts force on the table due to gravity.

Reaction- the table exerts equal force on the book.

d) Action - the fuel burns and releases large amount of force in opposite direction of the movement of rocket.

Reaction- the burnt fuel exerts equal pressure on the rocket and rocket move in upward direction.

e) Action - the person exerts force on the floor

Reaction - the floor exerts equal force on the legs of person due to which he move forward.

f) Action - the train collides to stationary train by applying large amount of force due to it's large momentum.

Reaction - the stationary train also exerts equal force on the moving train which opposes the motion of the moving train.

Why a man pulling a bucket of water from a well, suddenly fell down on breaking down of rope ?

Initially, the man was pulling the rope in upward direction and a reaction force is exerted by the bucket. After breaking of rope, the force acting on man by the rope disappears. so, due to the disappearance of this force, by law of inertia he fells down. Heavier the buckets, heavier the thrust the man will feel.

In water / oil tankers some space is left empty at the top while filling them. Explain, why ?

When a moving water/oil tanker suddenly stops, then due to the inertia of motion, the water/oil inside the tanker continues to be in a state of motion, and hence rushes in the forward direction. Similarly, when a stationary oil/water tanker is set into motion, the oil/water rushes in the backward direction. Therefore, to prevent any overflow of the oil/water due to sudden start or stop of the tanker, some space is left empty at the top while filling them.

Why a standing passenger was thrown in the forward direction when a running bus stops suddenly?

Passengers standing or sitting in a running bus share the motion of the bus. This means the passengers in a running bus are also moving at the same speed in the same direction. When the running bus stops suddenly the lower part of the passenger's body comes to rest, while the upper part continues to remain in motion. As a result, the passengers are thrown in the forward direction.

A person is standing on ice at the centre of a frozen lake. What should he do to reach the shore ?

He should exert a push on the ice, (taking help if possible, of a stick or pole) backwards. And according to newton third law the ice will apply equal and opposite force in forward direction on person.So due to the reaction he will be pushed forward and thus slipping on the surface of the ice, he will reach the shore.

Give scientific reason .
Even though the magnitudes of action force and reaction force are equal and their direction are opposite, their effects do not get cancelled.

When two bodies interact then action force and reaction comes in picture. Even though their magnitude are same and their direction are opposite, their effect do not get cancelled because these action and reaction forces do not act on the same body.

Why does it happen?
If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

If a stationary bus suddenly speeds up, passengers are thrown in the backward direction. This is because initially the whole body of a passenger inside the bus was in the state of rest. But, when the bus suddenly starts or speeds up, the lower half of the passenger's body comes in motion in the forward direction but the upper half still remains at rest due to inertia of rest. Hence, the passengers are thrown backward when a stationary bus suddenly speeds up.

What is inertia ?

The property by virtue of which a body tends to oppose any change in its position of rest or of uniform motion is called inertia.

What do you mean by inertia ?

Newton’s first law of motion defines inertia as the inherent property of a body by virtue of which it resists any change in its state of rest or of uniform motion in a straight line on its own. This law is also known as the law of inertia.

Justify the statement "Mass of a body is the measure of its inertia".

As we know that it is very difficult to change the state of rest or of uniform motion of a heavier body than that of a lighter one. So, mass is a measure of inertia.

Calculate the mass of the body, when a force of $525 \,N$ produces an acceleration of $3.5 \,m / s^2.$

We are given that force

$(F) = 525 \,N$
Acceleration

$(a) = 3.5 \,m/s^2$
Using,

$F = ma$

$\dfrac{F}{a} = \dfrac{525}{3.5} = 150 \,kg$

Define balanced and unbalanced forces. Explain with the help of figure that only unbalanced force produce motion in the body.

Balanced forces: When a number of forces acting on a body do not cause any change in its state of rest or of uniform motion along a straight line then the forces are said to be balanced forces. In other words, a body is said to be underbalanced forced when the resulting force acting on the body is zero.
The balanced forces:

$\cdot$ Cannot set any stationary body into motion.

$\cdot$ May change the shape and size of soft objects.

$\cdot$ Cannot change the speed/velocity of a moving body.
Unbalanced forces:
When the resultant of all the forces acting on a body is not zero, then forces are called unbalanced forces.
Example:

$\cdot$ Game of tug of war: When the forces exerted by both the teams are equal, then the rope does not move. But, if the force applied by team A is greater than team B, then the rope, as well as members of the weaker team, i.e., B, will be pulled towards A. The unbalanced force can (a) Set a stationary body in motion.

$\cdot$ Set a moving body at rest.

$\cdot$ Change the direction of motion.

A body is acted upon by two forces, but it does not move.
What can you say about the two forces ?

The two forces must be equal in magnitude and in opposite direction. So, they balance each other. As there is no unbalanced force, the body does not move.

What are the directions of action and reaction in Newtons third law of motion?

Opposite to each other but act on different bodies. The law of action-reaction (Newton's third law) explains the nature of the forces between the two interacting objects. According to the law, the force exerted by object 1 upon object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 upon object 1

State and explain Newton's second law of motion. Derive the expression $F = ma.$

Newton’s second law of motion states that the force exerted by a body is directly proportional to the rate of change of its momentum. For a body of mass ‘m’, whose velocity changes from u to v in time t, when force ‘F’ is applied.

$F \propto \dfrac{Change \,in \,momentum}{Time}$

$F \propto\dfrac{mv - mu}{t}$

$F \propto m \left ( \dfrac{v - u}{t} \right )$

$\Rightarrow F \propto ma \Rightarrow F = kma \left ( \because a = \dfrac{v - u}{t} \right )$

$\Rightarrow F = ma (\because k = constant = 1)$

Why do passengers jumping out of a rapidly moving vehicle, fall forward ? Give reason.

A passenger travelling in a bus/train share the motion of the bus/train. Thus the passenger's body is moving in the direction of the bus with the same speed. When the passenger jumps out of a fast-moving bus/train, his feet will come to rest on touching the ground, whereas the upper part of his body continues to move forward, due to the inertia of motion. As a result, if he does not run forward he will fall with his face in downwards direction.

A book is placed on a table. The weight of the book and the normal force by table on the book are equal in magnitude. Are they acting in the same direction?

The weight of the book is acting vertically downward but the normal force on the book is acting vertically upward. So, they are acting in the opposite direction.
1752347_1832392_ans_744874a18dbc43f9862d7406fd15ff06.png

1752347_1832392_ans_744874a18dbc43f9862d7406fd15ff06.png

Define inertia.

The tendency of a body to oppose any change in its state of rest or of uniform motion is called inertia.

What does rate of change in momentum give?

Rate of change of momentum gives the value of resultant force acting on a body.

$F=\dfrac{m(v-u)}{t}$ .

Two masses are In the ratio 1:What is the ratio of their Inertia?

Inertia of a body is directly proportional to its mass. Therefore the ratio of their inertia is their inertia is also in the ratio 1:2.

According to Newtons third law of motion, in a game of tug, each team pulls the opposing team with equal force. Then, why a team wins and the other one loses?

In the game of tug, till both the teams pull the rope with the same force, the net force on the system remains zero. As the force applied by one team increases than the other one, the whole system starts moving in the direction of the net force. Thus one team wins and the other one loses.

Passengers in buses tend to fall back as it accelerates. Why?

Due to inertia, the passengers tend to continue their state of rest, when the bus moves by accelerating.

An object experiences a zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no provide a reason.

Explanation :

$\bullet$ Yes, an object may travel with a non-zero velocity even when the net internal force on it is zero.

$\bullet$ An object moving in some direction with constant velocity will continue in it's state of motion as long as there are no external unbalanced forces acting on it.

$\bullet$ In order to change the motion of the object, some external unbalanced force must act upon it.

$\bullet$ A raindrop falls down with a constant velocity. The weight of the drop is balanced by the up thrust and the velocity of air. The netforce on the drop is zero.

Calculate the force required to change the velocity of an object of mass $4\,kg$ from $2\,ms^{-1}$ to $4\,ms^{-1}$ in one second.

$F = ma$

$m = 4\,kg$

$time$

$taken=1s$

$v_1 =2\,ms^{-1}$ Initial velocity

$v_2 =4\,ms^{-1}$ Final velocity

$F = ?$

$acceleration=a=\dfrac{change \quad in \quad velocity}{time}=\dfrac{v_2-v_1}{t}=\dfrac{4-2}{1}=2m/s^2$

$F = 4\times a$

$F = 4 \times 2 =8N$

Fill in the blanks:
The inertia of an object is proportional to its .................

The inertia of an object is proportional to its mass.

Inertia is the resistance of any physical object to any change in its velocity. This includes changes to the object's speed or direction of motion. An aspect of this property is the tendency of objects to keep moving in a straight line at a constant speed when no forces act upon them.

From a rifle of mass $4 kg$ , a bullet of mass $50 g$ is fired with an initial velocity of $35ms^{-1}$ , calculate the initial recoil velocity of the rifle.

Mass of the gun

$M = 4 kg$ .

Mass of bullet

$M = 50 g = 50 * 10^{-3}kg$ .

Initial velocity of the bullet

$v = 35ms^{-1}$

Let recoil velocity of the gun be

$vms^{-1}$

Before firing the bullet both gun and bullet were in rest so total momentum of gun and bullet is

zero,

After firing,

Momentum of bullet

$= mv$ ,

Momentum of gun

$= Mv$

Total momentum of bullet and gun after firing

$= mv + Mv$

Since there is no external force applied on the system.

So total momentum after firing = total momentum before firing,

$mv + Mv = 0$

$Mv = -mv$

$f_{i}$

$v=\frac{mv}{M}=\frac{50*10^{-3}*35}{4}=-0.44ms^{-1}$ .

Here negative sign shows that there recoil velocity of the gun is in the direction opposite to the velocity of the bullet.

Derive an expression for recoil velocity of gun.

Let

$m_g$ be mass of gun,

$\vec{v_g}$ = recoil velocity of gun,

$m_b$ be mass of Bullet

$\vec{v_b}$
Initial momentum = 0,
Final momentum =

$m_g \vec{v_g} + m_b \vec{v_b}$
By law of conservation of momentum

$0= m_g \vec{v_g} + m_b \vec{v_b}$

$\Rightarrow \vec{v_b} = -\Big(\dfrac{m_b\vec{v_b}}{m_g}\Big)$

What are balanced forces ?

If two or more forces acting on the same object keep the object in equilibrium position or in the same state, then the forces are balanced forces.

A hammer of mass $500\ g$ moving at $50\ ms^{-1}$ strikes a nail. The nail stops the hammer in a very short time of $0.61\ s$ . What is the force of the nail on the hammer?

Given,

Mass of the hammer,

$m=500\ g=0.5\ kg$

Initial velocity,

$u=50\ m/s$

Final velocity,

$v=0$

Time taken,

$t=0.61\ s$

Acceleration,

$a$

Using Newton's first equation of motion,

$v=u+at$

$\implies a=\dfrac {v-u}{t}$

$a=\dfrac {0-50}{0.61}$

$a=82\ m/s^2$

We know,

Force,

$F=ma$

$F=0.5\times82$

$F=42\ N$

Find out reasons for the Situations
When a running bus is suddenly stopped, passengers, standing in the bus show a tendency to fall forward.

When the bus was running, the passengers in the bus were also moving along with the bus. Hence they had inertia of motion. Now if the bus is suddenly stopped, the inertia of passengers does not change immediately. Due to this fact, the passengers lean forwards as their bodies try to maintain the inertia of motion.

When a carpet is beaten with a stick, dust comes out of it. Explain.

Explanation:

$\bullet$ According to Newton’s first law of motion, if a body is at rest or moving in a straight line with constant speed, then it will remain at rest or keep moving unless it is acted upon by a force. This phenomenon of a body tends to remain in rest or moving in straight line is called inertia of the body.

$\bullet$ When the carpet is not beaten or at rest dust particles also in rest with the carpet. When the carpet is beaten an impulsive force acted upon the carpet which make the carpet to be in motion, but dust particles do not come under the influence of this force, so they tends to remain at rest. Thus dust particles be separated from the carpet and comes out of it.

Fill in the blanks with suitable words.
Force is the product of _ and _

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A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of $400m$ in $20 s$ . Find its acceleration. Find the force acting on it if its mass is $7tons$ . (Hint: $1 ton = 1000 kg$ .)

The truck starts from rest, so initial velocity

$u = 0$ ,

Distance

$s = 400 m$ ,

$t = 20s$ ,

$m = 7 tons = 7 * 1000 = 7000 Kg$ .

From 2nd equation of motion,

$s=ut+\frac{1}{2}at^{2}$

$400=0*20+\frac{1}{2}*a*(20)^{2}$

$400=200a$

$f_{i}$

$a=2ms^{-2}$ .

From Newton’s 2nd law of motion,

force acting on the Duck,

$= 7000 \times 2$

$= 14000 N$

$= 1.4 \times 10^{4} N$ .

Find out reasons for the situation:
Place some carom board coins in a pile. Using the striker, strike out the coin at the bottom. What do you observe? What is the reason?

Only the coin at the bottom is thrown away. Others will remain in the previous stage.

This is because inertia of rest. The upper coins remained at rest and the coins at bottom by the sudden hit (Force) by striker moves.

Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Because even though bus comes to rest, luggage's are in motion, They fall down. So, it is tied with a rope to prevent falling.

According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probable not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.

According to the third law of motion when we push an object, the object pushes us back with an equal and opposite force. These two forces are called as action-reaction pair.

The student’s justification is incorrect since action and reaction always operate on different bodies and hence cannot cancel each other. When two equal and opposite forces act on the same body, they cancel each other.

A truck parked on the side of the road has a heavy weight and generates greater friction between the tyres and the ground than the force applied. As a result, the truck will only move if a force greater than the friction force is applied to it.

Expand the table by finding more situation from daily life.
Inertia of rest Inertia of motion
$\bullet$ When the branch of a mango tree is shaken, mangoes fall just as when the branch starts moving. $\bullet$ A running athlete cannot stop himself abruptly at the finishing line in a race.
$\bullet$ $\bullet$

Inertia of rest	Inertia of motion
$1$ When the branch of a mango tree is shaken, mangoes fall.	$1$ A running athlete cannot stop himself abruptly at the finishing point.
$2$ When the carpet is trapped, dust particle scatters.	$2$ A man stepping down from a slowly moving bus stops after few steps of running.
$3$ When the coin on a Cardboard placed over a glass is struck, it falls into the glass	$3$ In hammer throw, before the hammer islet go off, it is whirled along a circular path.

It is more difficult to roll a drum filled tar than an empty drum.
Which of two has a greater mass?

The mass of drum filled with tar will be greater.

Bacause it is inertia which causes the restrictions for movement and inertia is depends on mass.

It is more difficult to roll a filled drum than an empty drum.
Which has greater inertia?

Inertia is the tendency of an object to resist any change in its state of motion/rest. It is directly proportional to the mass of the object.

Since, a filled drum has a greater mass than an empty drum. Hence, it has a greater inertia and it is more difficult to roll a filled drum.

Find out the reasons
A running elephant cannot change its direction suddenly.

Mass of elephant is greater. So inertia of motion also is greater. Also, it cannot be able to change its direction suddenly.

During a pole vault jump, the impact is reduced by falling on a foam bed. Give reason.

When the change in momentum

$\Delta P$ is a constant, the force acting on a body will be inversely proportional to the time taken

$\Delta t$ .

We know

$F=\dfrac{\Delta P}{\Delta t}$

As time

$\Delta t$ increases, the force acting decreases, and as time decreases, the force acting increases. On a foam bed, the body takes some time to come to rest as the bed is pressed. So, the impact is reduced.

It is dangerous for loaded vehicles to negotiate a curve in the road without reducing speed. What is the reason?

Loaded vehicles possess more inertia of motion because of mass contain in it. When it take turn, direction of motion changes but its inertia will resist this changes and try to keep its motion in initial direction. So there is a chances to skid or toppling the vehicles on rod. To avoid such things, need to reduce speed.

Examine the following situations and complete the table.
Situation Action Reaction
$\bullet$ A man jumps from a boat to the shore. The man exerts a force on the boat. THe boat moves backward. The boad exerts an equal force on the man in opposite direction. THe man lands on the shore.
$\bullet$ A bullet is fired from a gun
$\bullet$ A boat is rowed

Situation	Action	Reaction
$1$ A man jumps from a boat to the shore.	The man exerts a force on the boat. THe boat moves backward.	The boad exerts an equal force on the man in opposite direction. THe man lands on the shore.
$2$ A bullet is fired from a gun	the bullet exert a force on the gun so it moves backward	The gun exerts an equal force to the bullet, so it moves forward
$3$ A boat is rowed.	The man applied force on the water	The boat moves forward.

Conclusions :

$\bullet$ As a result of the applied force by a second body to a body, reaction will occur at the second body.

$\bullet$ Action and reaction are equal and opposite.

$\bullet$ Sice action and reaction takes place in two different bodies, they do not cancel each other.

Give reasons:
When a bullet is fired from a gun, the gun recoils.

The gun recoil due to the reaction force applied by the shot to the gun. Forward movement of the shot is action and the Backward movement of the gun is reaction.

Write down the action and reaction while we are walking on a floor?

When we are walking on a floor, we apply a force on the floor from our foot in backward direction. This is action. The floor applies a force in the opposite direction(forward). This is reaction which pushes us forward.

Observe the figure and answer the question.
When the card is suddenly struck off, what happens to the coin? Explain.

The coin falls into the glass due to inertia of rest. As according to newton's first law of motion, a body will remain at rest if it is at rest and in motion if it is in motion until and unless external unbalanced force is applied on it.

Give reasons:
When a bus at rest suddenly moves forward, the passengers, standing in the bus, fall backward.

An opposite experience is encountered when we are standing in a bus and the bus begins to move suddenly. Now we tend to fall backward. This is because the sudden start of the bus brings motion to the bus as well as to our feet in contact with the floor of the bus. But the rest of our body opposes this motion because of its inertia.

On the basis of Newton's third law of motion, explain the source of force that helps to propel a rocket upward.

The source of force in rocket propulsion are:

$\bullet$ It is due to the reaction force by the escaping gas from rocket.

$\bullet$ Escaping gases from the roket is action.

$\bullet$ The force exerted escaping gases on the rocket is reaction.

A person holds a ball in her hand.
(a) Identify all the external forces acting on the ball and Newton’s third law reaction force to each one.
(b) If the ball is dropped, what force is exerted on it while it is falling? Identify the reaction force in this case. (Ignore air resistance)

(a) i. Action force: Earth pulls the ball downwards with the force of gravity

$F_g$

Reaction force: Ball pulls the earth upwards with the force of gravity

$F_g$

ii. Action force: Normal force that the ball exerts on the hand

$mg$ (downward)

Reaction force: Normal force that the hand exerts on the ball

$N$ (upward)

(b) Action force: Earth pulls the ball downwards with the force of gravity

$F_g$

Reaction force: Ball pulls the earth upwards with the force of gravity

$F_g$

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A rubber ball is dropped onto the floor. What force causes the ball to bounce?

The molecules of the floor resist the ball on impact and push the ball back, upward. The actual force acting is due to the forces between molecules that allow the floor to keep its integrity and to prevent the ball from passing through. Notice that for a ball passing through a window, the molecular forces weren’t strong enough.

A passenger sitting in the rear of a bus claims that she was injured when the driver slammed on the brakes, causing a suitcase to come flying toward her from the front of the bus. If you were the judge in this case, what disposition would you make? Why?

The use here is made of the Newton's first law or law of inertia, according to which an object in a state of rest/motion, will continue to be in a state of rest/motion, until acted acted upon by an external force.

First ask, “Was the bus moving forward or backing up?” If it was moving forward, the passenger is lying. A fast stop would make the suitcase fly toward the front of the bus, not toward the rear. If the bus was backing up at any reasonable speed, a sudden stop could not make a suitcase fly far. Hence, the passenger is lying.

Identify actionreaction pairs in the following situations: (a) a man takes a step (b) a snowball hits a girl in the back (c) a baseball player catches a ball (d) a gust of wind strikes a window.

(a) As a man takes a step, the action is the force his foot exerts on the Earth; the reaction is the force of the Earth on his foot. (b) The action is the force exerted on the girl’s back by the snowball; the reaction is the force exerted on the snowball by the girl’s back. (c) The action is the force of the glove on the ball; the reaction is the force of the ball on the glove. (d) The action is the force exerted on the window by the air molecules; the reaction is the force on the air molecules exerted by the window. We could in each case interchange the terms “action” and “reaction.”

Can an object exert a force on itself? Argue for your answer.

An object cannot exert a force on itself, so as to cause acceleration. If it could, then objects would be able to accelerate themselves, without interacting with the environment. You cannot lift yourself by tugging on your bootstraps.

What forces cause (a) an automobile, (b) a propeller driven airplane, and (c) a rowboat to move?

(a) Friction, either static or kinetic, exerted by the roadway where it meets the rubber tires accelerates the car forward and then maintains its speed by counterbalancing resistance forces. Most of the time static friction is at work. But even kinetic friction (racers starting) will still move the car forward, although not as efficiently.
(b) The air around the propeller pushes forward on its blades. Evidence is that the propeller blade pushes the air toward the back of the plane.
(c) The water pushes the blade of the oar toward the bow. Evidence is that the blade of the oar pushes the water toward the stern.

State Newton's 2nd law of motion.

The second law of notion states that the rate of change of momentum of a body is directly proportional to the unbalanced external force acting on it.

Let a force

$F$ is acting on a body. Due to the force, the momentum of the body changes by

$\Delta P$ in time

$\Delta t$ .

The rate of change of momentum =

$\dfrac{\Delta P}{\Delta t}$

According to Newton's second law of motion:

$F\propto \dfrac{\Delta p}{\Delta t}$

Try the following experiment away from traffic where you can do it safely. With the car you are driving moving slowly on a straight, level road, shift the transmission into neutral and let the car coast. At the moment the car comes to a complete stop, step hard on the brake and notice what you feel. Now repeat the same experiment on a fairly gentle, uphill slope. Explain the difference in what a person riding in the car feels in the two cases. (Brian Popp suggested the idea for this question.)

Tramping hard on the brake at zero speed on a level road, you do not feel pushed around inside the car. The forces of rolling resistance and air resistance have dropped to zero as the car coasted to a stop, so the car’s acceleration is zero at this moment and afterward. Tramping hard on the brake at zero speed on an uphill slope, you feel thrown backward against your seat. Before, during, and after the zero speed moment, the car is moving with a downhill acceleration if you do not tramp on the brake.

A hunter has a machine gun that can fire 50 g bullets with a velocity of 150 m/s. A 60 kg tiger springs at him with a velocity of 10 m/s. How many bullets must the hunter fire per second into the tiger in order to stop him in his track?

Show that when the net force acting on a body is zero, it has no acceleration.

From Newton's second law, we get

$F=ma$ , where

$F$ is the force acting on an object of mass

$m$ . The acceleration of the object is

$a$ .

$F=0$ then

$a=0$ .

Hence, when the net force acting on an object is zero, it has no acceleration.

One or more external forces, large enough to be easily measured, are exerted on each object enclosed in a dashed box shown in above figure. Identify the reaction to each of these forces.

(a) Force exerted by spring on hand, to the left; force exerted by spring on wall, to the right.
(b) Force exerted by wagon on handle, downward to the left. Force exerted by wagon on planet, upward. Force exerted by wagon on ground, downward.
(c) Force exerted by football on player, downward to the right. Force exerted by football on planet, upward.
(d) Force exerted by small-mass object on large-mass object, to the left.
(e) Force exerted by negative charge on positive charge, to the left.
(f) Force exerted by iron on magnet, to the left.

A sharpshooter fires a rifle while standing with the but of the gun against her shoulder. If the forward momentum of a bullet is the same as the backward momentum of the gun, why isn't it as dangerous to be hit by the gun as by the bullet?

Though the momentum of both the object is the same but mass of the gun is large compared to the mass of the bullet hence it has a very small velocity. Also, the area of contact of the gun is very large compared to the bullet point area hence pressure exerted by the gun will be very small compared to pressure due to bullet.

$\text{Momentum} = \text{mass} \times \text{velocity}$

$\text{Pressure} = \dfrac{\text{Force}}{\text{area}}$

The force of 2N acting on the changes its velocity uniformly from 2 m/sec to 5 m/sec in 10 seconds. Calculate the mass of the body.

$\textbf{Hint:}$ Use equation of motion to obtain acceleration and use Newton’s second law to obtain mass of the body.

Step 1: Note the given values

$\text{Given,}$

$\text{Force, f=2N}$

$\text{Initial velocity, u=2 m/s }$

$\text{Final velocity, v=5 m/s}$

$\text{Time , t=10 s}$

$\textbf{Step 2: Finding acceleration of the Body}$

$\text{Using equation of motion:}$

$\text{v=u+at}$

$\text{Putting the values:}$

$5=2+a\times 10$

$a=(5-2)/10$

$a=0.3 m/s^{2}$

$\text{So,acceleration i.e }{a=0.3 m/s^{2}}$

Step 3: Finding mass of body

$\text{According to Newton's Second Law of Motion}$

$f=ma$

$m=\dfrac{f}{a}$

$m=\dfrac{2}{0.3}$

$m=6.66 Kg$

$\textbf{Hence,mass of the body is 6.66 Kg.}$

Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compare to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experience the same force and change in their momentum. Comment on these suggestions

Kiran's suggestion is not correct because momentum is always conserved
i.e. change in momentum of insect must be equal and opposite to that of motorcar.
Akhtar's suggestion is also not correct because how can he decide the velocity of motorcor was more than the velocity of insect.
Rahul's suggestion is correct ,according to newtons third law of motion- For every action, there is an equal and opposite reaction. so,insect and motorcar experience same force at same time.
we know,
Force

$=$ change in momentum per unit time
here time and force both are same. so,the change in momentum will be same
However, the insect dies, because it is unable to bear the large force and large change in momentum.

Horse cannot pull a cart and run in an empty space .

A cricketer moves his hand backward while holding a catch.

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When we shake the branch of tree its fruits and dry leaves falls.

when a tree is shaken it's fruits and leaves fall down due to the gravitational force exerted by earth.

All fruits and leaves do not fall when tree is shaken, mostly only those fruits and leaves are fallen that are ripped.

It's because their tissues has become weak.

Describe Newton's laws of motion on the basis of daily life examples. On the basis of the second law, find the relation between force, mass and acceleration.

Newton's first law of motion- According to this law, a body at rest or in uniform motion will continue to be in the state of rest or uniform motion unless it is compelled by an external force to change its state of rest or uniform motion.
Examples:

$\cdot$ An athlete runs for a certain distance before taking a long jump.

$\cdot$ A person sitting / standing on a bus fall back when the bus suddenly starts moving forward.

$\cdot$ The carpet is beaten with a stick to remove the dust particles.
The second law of motion: The acceleration produced in the body by the action of a force acting on its directly proportional to the force and inversely proportional to the mass of the body. Mathematically,

$F = ma,$ where, F = force, m = mass of the body.
In other words, Newton's second law of motion can be stated as: The rate of change of momentum is equal to the force applied to the body and the change in momentum always takes place in the direction of the force.
Examples:

$\cdot$ A cricket player lowers his hands while catching the ball.

$\cdot$ A person falling on a cemented floor gets injured, but a person falling on a heap of sand does not get injured.
Third law of Motion: According to this law:
"For every action, there are an equal and opposite reaction" (Law of action and reaction) Examples:

$\cdot$ Gun gives a jerk to shoulder of the gunman on firing a bullet from it.

$\cdot$ Boatman pushes the river bank with a bamboo pole to take his boat into the river.

In Fig.5-44,elevator cabs A and B are connected by a short cable and can be pulled upward or lowered by the cable above cab A. Cab A has mass $1700kg$ ;cab B has mass $1300kg$ .A $12.0kg$ box of cat-nip lies on the floor of cab A.The tension in the cable connecting the cabs is $1.91 \times 10^4 N$ .What is the magnitude of the normal force on the box from the floor.

Applying Newton’s second law to cab B (of mass m) we have

$a =\frac{T}{m}- g= 4.89 \, m/s^2$
Next, we apply it to the box (of mass mb) to find the normal force:

$F_N = mb (g+a)= 176 \, N$ .

Your hands are wet, and the restroom towel dispenser is empty. What do you do to get drops of water off your hands? How does the motion of the drops exemplify one of Newtons laws? Which one?

When the hands are shaken, there is a large acceleration of the surfaces of the hands. If the water drops were to stay on the hands, they must accelerate along with the hands. The only force that can provide this acceleration is the friction force between the water and the hands. (There are adhesive forces also, but let’s not worry about those.) The static friction force is not large enough to keep the water stationary with respect to the skin at this large acceleration. Therefore, the water breaks free and slides along the skin surface. Eventually, the water reaches the end of a finger and then slides off into the air. This is an example of Newton’s first law in action in that the drops continue in motion while the hand is stopped

Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.

$\textbf{Hint:}$ Apply Newton's third law of motion.

Explanation :

When a fireman holds a horse, while is ejecting large amount of water at a high velocity, then a reaction force is exerted on him by the ejecting water in the backward direction. This is because of Newton's third law of motion. As a result of the backward force, the stability of the fireman decreases. Hence, it is difficult for him to remain stable while holding the horse.

In the motion picture It Happened One Night (Columbia Pictures, 1934), Clark Gable is standing inside a stationary bus in front of Claudette Colbert, who is seated. The bus suddenly starts moving forward and Clark falls into Claudette’s lap. Why did this happen?

When the bus starts moving, the mass of Claudette is accelerated by the force of the back of the seat on her body. Clark is standing, however, and the only force on him is the friction between his shoes and the floor of the bus. Thus, when the bus starts moving, his feet start accelerating forward, but the rest of his body experiences almost no accelerating force (only that due to his being attached to his accelerating feet!). As a consequence, his body tends to stay almost at rest, according to Newton’s first law, relative to the ground. Relative to Claudette, however, he is moving toward her and falls into her lap.

State the law of action and reaction. Explain with a suitable and necessary diagram that
Forces of action and reaction operate on two different objects.
Forces of action and reaction are equal, but they act in opposite direction.

When a force acts on a body, then it is called the action. The force produced as a result of action acts on another object but in opposite direction is called reaction. Newton formulated a law in relation to action and reaction, which is called Newton’s third law of motion.According to this law, for every action, there is an equal and opposite reaction. Action and reaction always operate on different objects but in opposite directions.
(1) Action and reaction act on two different objects.
Experiment: Take an oil funnel and fill half of it with water and fix it on a small boat, as shown in the figure. The funnel is heated at the bottom with the help of a lamp. When steam begins to come out from the jet fastly, the boat experiences a force in a backward direction, and it begins to move in a direction opposite to the direction of the outgoing steam the body. It can also be stated as: The rate of change of momentum is equal to the force applied to the body and the change in momentum always takes place in the direction of the force. From this experiment, it is clear that the force (action) due to the outgoing steam acts in a forward direction and force of reactions acts, i.e., moving boats in the opposite direction. This proves that action and reaction act on two different objects.
(2) Experiment: Take two similar spring balances, as shown in the figure. Fix one spring balance to a rigid support and then fix the hook of another spring balance in its hook, and stretch (pull) the whole system gently. Observe the readings of both the spring balances. We see that the reading of both the spring balances is exactly equal. If one balance reads 5 kg, the other also read 5 kg. Obviously, the tension of the first on the second spring balance is exactly equal the tension of the second on the first spring balance. This means that the action and reaction forces are equal and opposite.

Inertia of rest	Inertia of motion
$\bullet$ When the branch of a mango tree is shaken, mangoes fall just as when the branch starts moving.	$\bullet$ A running athlete cannot stop himself abruptly at the finishing line in a race.
$\bullet$	$\bullet$

Force And Laws Of Motion - Class 9 Physics - Extra Questions

Selvi placed her purse on the passengers seat of her car when she drove to work. By the time she reached her office, her purse had fallen on the floor in front of the passengers seat. Why did this happen? Explain.

A shopping cart has a mass of 65 kg65\ kg. In order to accelerate the cart by 0.3 ms−20.3\ ms^{-2} what force would you exert on it?

What is the force called which produces an acceleration of 1m{s}^{2}1m{s}^{2} in a body of mass 1kg1kg?

Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. It is based on which law?

State Newton's second law of motion.

Why is it easier to maintain the motion than to start it?

Do we apply a force on the ground while walking? (Yes/No)

To move a boat ahead in water, the boatman has to push the water backwards by his oar. Explain.

Newton's Third Law says that every action has an equal and opposite reaction. State True or False.

A heavy car A of mass 2000 kg travelling at 10 m/s has a head-on collision with a sports car B of mass 500 kg.If both cars stop dead on colliding,what was the velocity of car B?

Explain why,when a fireman directs a powerful stream of water on a fire from a hose pipe, the hose pipe tends to go backward.

Differentiate between Mass and inertia

Give reason: Sparks produced in a grinding stone move tangentially.

A bullet of mass 50\ g50\ g moving with an initial velocity of 100\ m/s100\ m/s, strikes a wooden block and comes to rest penetrating a distance 2\ cm2\ cm in it. Calculate the initial momentum of the bullet.

A motorcar with mass 1200kg1200kg can be pushed by three persons to produce an acceleration of 0.2ms^{-2}0.2ms^{-2} .With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort).

Find out reasons for the situation:Place a small brick on a plank. When the plank is pulled suddenly the brick remains in the same position as before.

You are hurt when you kick a stone. Why?

A body of mass 25 kg25 kg has a momentum of 125 kg m/s125 kg m/s. Calculate the velocity of body

On what factor does the inertia of a body depend? Which has more inertia, a cricket ball or rubber of the same size?

If action is always equal to the reaction, explain how a horse can pull a cart.

Mention the condition when an object in motiona) can be considered point object b) can not be considered point object

Weight of an object is the _____________ force of the ____________ acting on an object.

A man pushes a wall with force F = 10\ NF = 10\ N towards the east. What force is exerted by the wall on the man?

What happens to the passengers travelling in a bus when the bus takes a sharp turn? Give reason for your answer.

What type of forces-balanced or unbalanced-act on a rubber ball when we press it between our hands? What effect is produced in the ball?

Two bodies A and B of same mass are moving with velocities v and 2v respectively. Compare their inertia.

A body of mass m moving with a velocity v is acted upon by a force. Write expression for change in momentum when v << cv << c but m does not remain constant. Here cc is the speed of light.

Two equal and opposite forces act on a stationary body. Will the body move? Give reason to your answer.

Why does a person fall when he jumps out from a moving train?

Name the two factors on which the force needed to stop a moving body in a given time, depends.

State Newton's second law of motion. What information do you get from it?

"The action and reaction both act simultaneously." Is this statement true?

If a man jumps out from a boat, the boat moves backward. Explain.

Greater the mass, greater is the inertia of the body. Type 1 for true and 0 for false

'More the mass, the more difficult it is to move the body from rest'. Type 1 for true and 0 for false

Comment on the statement 'the sum of action and reaction on a body is zero'.

Can you identify the action and reaction forces in the case of an object falling under gravity? Neglect air resistance

To cause acceleration in an object the necessary factor is unbalanced _____________ .

If two horizontal forces F_1F_1 and F_2F_2 of the unequal magnitude act on a body in opposite directions then find the net force acting on the body.

Distinguish between balanced and unbalanced forces.

Define spring balance.

Correct the mistakes, if any, in the following statements.i) One newton is the force that produces an acceleration of 1\ ms^{-2}1\ ms^{-2} in an object of 11 gram mass.ii) Action and reaction always act on the same body.

Why does a fielder in the game of cricket pull his hands back when he catches a ball?

What happens when you shake a wet piece of cloth ? Explain.

Explain rocket propulsion.

By a bullet fire by a gun is more dangerous than a bullet fire with medium speed?

The mass of a cannon is 500\ kg500\ kg and it recoils with a speed of 0.25\ m/s0.25\ m/s. What is the momentum of the cannon?

Guess and write, a heavy truck causes more damage to the wall than a bicycle if both hit a wall with same speed. Why?

Which would require a greater force-accelerating a 11 kg mass at 1010 m/s^{-2}m/s^{-2} or a 22 kg mass at 4 m/s^24 m/s^2.

A person standing in a moving bus falls backwards when bus suddenly starts moving in forward direction. This happens due to

State the relation between force and momentum.

When a Carpet is beaten with a stick, dust comes out of it. Explain.

................ is a measure of the inertia of a body.

If action is always equal to reaction;explain

Fill in the blanks:A moving football when kicked, its _________ changes.

Name the force which is responsible for changes in position or state of an object.

If the action is the cork being pushed out to the force exerted by the steam on it, what is the reaction?

How much force is needed to increase velocity of a body of 4kg4kg mass by 1.5m/s1.5m/s every second?

Force produced by shooting makes the bullet to move forwards, also equal force in opposite direction is applied which recoils gun backwards. It is used to refill gun in semi automatic weapons. State True or False.

Give two examples each of situation in which you push or pull to change the state of motion of an object.

Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Give reasons for the following;An athlete in the long jump runs a distance before taking the leap.

Explain why some of the leaves may get detached from a tree if we vigorously shake it branch.

Give reasons for the followingUsually a fielder in a cricket match, takes a catch by moving his hands in the direction of motion of cricket ball.

State any two illustrations for Newtons third law of motion.

From the following statements write down which is not applicable to mass of an object.(a) It is a fundamental quantity.(b) It is measured using physical balance.(c) It is measured using spring balance.

Why does a boxer always move along the direction of the punch of the opponent?

Write the principles that are used in rocket propulsion.

The mats used in gyms and the padding used in sports uniforms are made up of soft substances. Why are rigid materials not used?

Two equal forces to P and 2P act on a particle.If the first be doubled and the second be increased by 20N, the direction of the resultant is unaltered.Find the value of P.

Which would require a greater force?Accelerating a 5\ kg5\ kg object at 3\ m/s^23\ m/s^2, or an 8\ kg8\ kg object at 2\ m/s^22\ m/s^2?

Find F for body to be in equilibrium.

Action and reaction are equal and opposite. Why cannot they cancel each other?

Explain Newton's second law of motion with the help of an example?

A force of 12\ N12\ N pulls a block of mass 3.5\ kg3.5\ kg through a rope of mass 0.5\ kg0.5\ kg. The block is resting on a horizontal frictionless surface. What is the force of reaction exerted by the block on the rope?

A hockey ball of mass 200g is traveling at 10 m/s and is struck by a hockey stick, so as to return it along its original path with a velocity of 50 m/s. Calculate the change of momentum occurred in the motion of hockey ball by the force applied by the hockey stick.

Guess and write why does pole vault jumper land on thick mats of foam?

Write and explain Newton's second law of motion.

A shopping cart has a mass of $65\ kg$ . In order to accelerate the cart by $0.3\ ms^{-2}$ what force would you exert on it?

What is the force called which produces an acceleration of $1m{s}^{2}$ in a body of mass $1kg$ ?

Newton's Third Law says that every action has an equal and opposite reaction.

State True or False.

A bullet of mass $50\ g$ moving with an initial velocity of $100\ m/s$ , strikes a wooden block and comes to rest penetrating a distance $2\ cm$ in it. Calculate the initial momentum of the bullet.

A motorcar with mass $1200kg$ can be pushed by three persons to produce an acceleration of $0.2ms^{-2}$ .With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort).

Find out reasons for the situation:
Place a small brick on a plank. When the plank is pulled suddenly the brick remains in the same position as before.

A body of mass $25 kg$ has a momentum of $125 kg m/s$ . Calculate the velocity of body

Mention the condition when an object in motion
a) can be considered point object
b) can not be considered point object

Weight of an object is the _ force of the acting on an object.

A man pushes a wall with force $F = 10\ N$ towards the east. What force is exerted by the wall on the man?

Two bodies A and B of same mass are moving with velocities v and 2v respectively. Compare their
inertia.

A body of mass m moving with a velocity v is acted upon by a force. Write expression for change in momentum when $v << c$ but m does not remain constant. Here $c$ is the speed of light.

If two horizontal forces $F_1$ and $F_2$ of the unequal magnitude act on a body in opposite directions then find the net force acting on the body.

Correct the mistakes, if any, in the following statements.
i) One newton is the force that produces an acceleration of $1\ ms^{-2}$ in an object of $1$ gram mass.
ii) Action and reaction always act on the same body.

The mass of a cannon is $500\ kg$ and it recoils with a speed of $0.25\ m/s$ . What is the momentum of the cannon?

Which would require a greater force-accelerating a $1$ kg mass at $10$ $m/s^{-2}$ or a $2$ kg mass at $4 m/s^2$ .

Fill in the blanks:
A moving football when kicked, its _________ changes.

How much force is needed to increase velocity of a body of $4kg$ mass by $1.5m/s$ every second?

Force produced by shooting makes the bullet to move forwards, also equal force in opposite direction is applied which recoils gun backwards. It is used to refill gun in semi automatic weapons.
State True or False.

Give reasons for the following;
An athlete in the long jump runs a distance before taking the leap.

Give reasons for the following
Usually a fielder in a cricket match, takes a catch by moving his hands in the direction of motion of cricket ball.

From the following statements write down which is not applicable to mass of an object.
(a) It is a fundamental quantity.
(b) It is measured using physical balance.
(c) It is measured using spring balance.

Which would require a greater force?
Accelerating a $5\ kg$ object at $3\ m/s^2$ , or an $8\ kg$ object at $2\ m/s^2$ ?

A force of $12\ N$ pulls a block of mass $3.5\ kg$ through a rope of mass $0.5\ kg$ . The block is resting on a horizontal frictionless surface. What is the force of reaction exerted by the block on the rope?

(a). Name the forces acting on a plastic bucket containing water held above ground level in your hand.

(b). Discuss why the force acting on the bucket do not bring change its state of motion ?

(c). Why do porters place a round of cloth on their heads when they have to carry heavy loads?

A 10 g bullet is shot from a 5 kg gun with a velocity of 400 $m{s^{ - 1}}$ .What is the speed of recoil of the gun?

An object of mass $10\ kg$ is moving with an initial velocity of $2\ ms^{-1}$ . To make it come to rest in one second, what should be the unbalanced force?

The velocity of a body of mass 10 kg increases from 4 m/s to 8 m/s when a force acts on it for 2 s.
a) what is the momentum before the force acts?
b) What is the momentum after the force acts?
c) what is the gain momentum per second?
d) What is the value of the force?

A gun of mass 3 kg fires a bullet of mass 30 g.The bullet takes 0.003 s to move through the barrel of the gun and acquires a velocity of 100 m/s.Calculate:
i) the velocity with which the gun recoils.
ii) the force exerted on gunman due to recoil of the gun.

Two balls of the same size but of different material, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed?
Give reason for your answer.

How can you feel a force of $1$ N?

A bullet of mass $50 \ g$ moving with an initial velocity of $100 \ m/s$ , strikes a wooden block and comes to rest penetrating a distance $2 \ cm$ in it. Calculate final momentum of the bullet.

Name and state the action and reaction in the following cases:
(a) Firing a bullet from a gun,
(b) Hammering a nall,
(c) A book lying on a table,
(d) A moving rocket,
(e) A person walking on the floor,
(f) A moving train colliding with a stationary train.

Give scientific reason .
Even though the magnitudes of action force and reaction force are equal and their direction are opposite, their effects do not get cancelled.

Why does it happen?
If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

Calculate the mass of the body, when a force of $525 \,N$ produces an acceleration of $3.5 \,m / s^2.$

A body is acted upon by two forces, but it does not move.
What can you say about the two forces ?

State and explain Newton's second law of motion. Derive the expression $F = ma.$