MCQ Questions for Class 9 Physics Force And Laws Of Motion Quiz 15

A person in a car tends to fall back when it suddenly starts. It is due to

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Inertia of rest
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Inertia of motion
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Inertia of direction
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None

Explanation

When the bus suddenly starts moving in forward direction. This happens due to. When a bus suddenly starts, the standing passengers fall backward in the bus.

It is due to inertia of rest.

Hence,

option

$A$ is correct answer.

Newton's

$2nd$ law explain us

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Measure of force
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Force,mass and acceleration
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Nature of force
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Types of force

Explanation

Newton's second law of motion pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object.

Hence,

option

$A$ is correct answer.

A force of 200 N is exerted on an object of mass 40 kg that is located on a sheet of perfectly smooth ice. If a second object identical to the first object is placed on top of the object. What acceleration would the 200 N force produce ?

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$5 \ m/ s^2$
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$2.5 \ m/ s^2$
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$10 \ m/ s^2$
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$12 \ m/ s^2$

Explanation

$\large\textbf{Step 1: For the First case}$

Given,

Force exerted $=$ $200N$

Mass of the object $= 40kg$

Ice sheet is perfectly smooth, Hence friction force is zero here

We know,

${\bf{Force}}{\rm{ }} = {\rm{ }}{\bf{Mass}}{\rm{ }} \times \;{\bf{Acceleration}}$

Or $F = ma$

Or Acceleration, $a = \dfrac{F}{m}$ $=$ $\dfrac{{200}}{{40}} = 5m/{s^2}$

$\large\textbf{Step 2: For the second case}$

Total mass $=$ $40+40$ $=$ $80 kg$

Force $=$ $200 N$

So,

Or Acceleration, $a' = \dfrac{F}{m} = \dfrac{{200}}{{80}} = 2.5m/{s^2}$

$\textbf{Hence, in the second case the acceleration is } 2.5m/s^2$

Rahul is traveling in a bus on a straight road facing in the direction of the motion of the bus. If the bus starts accelerating in the same direction then Rahul will fall

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Forward
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Backward
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Rightward
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Leftward

A boy who normally weigh

$200 \ N$ on a spring scale suddenly jump upwards. The scale reading jumps to

$400 \ N$ . What is boy's maximum acceleration upwards?

$(g=9.8m/s^2)$

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$2m/s^2$
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$3m/s^2$
0%
$9.8m/s^2$
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$4.67m/s^2$

Explanation

Given,

Weight

$W= 200 \ N$

$g=9.8m/s^2$

The Normal weight of the boy,

$W=mg$

$m=\dfrac{W}{g}=\dfrac{200}{9.8}$

$m=20.40kg$

When a boy suddenly jump upward with acceleration

$a$ , then reading is

$400N$

This reading is nothing but the normal force on the boy in upward direction.

So two forces act on the boy, one is

$mg$ downwards and the other is

$N=400 \ N$ upwards. The resultant of these forces gives an acceleration

$a$ in the upward direction.

By applying Newton's second law,

$N-mg=ma$ (taking up as positive)

$400-20.40\times 9.8=20.40a$

$20.40a=400-199.92$

$20.40a=200.08$

$a=\dfrac{200.8}{20.40}$

$a=9.8m/s^2$

The correct option is C.

1546528_1390963_ans_888be3a13e2a4f189f7162341478fc12.png

A car is moving with

$15\ ms^{-1}$ whose mass is

$1500\ kg$ . Find the momentum of the car.

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$22500\ kg\ ms^{-1}$
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$2500\ kg\ ms^{-1}$
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$500\ kg\ ms^{-1}$
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$11500\ kg\ ms^{-1}$

For every action there exists an equal and opposite reaction

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True
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False

A trolley of mass

$300 kg$ carrying a sandbag of

$25 kg$ is moving uniformly with a speed of

$27 km/h$ on a frictionless track. After a while, sand starts leaking out of a hole on the trolley's floor at the rate of

$0.05 kg/s$ . The speed of the trolley after the entire sand bag is empty is

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$27 km /hr$
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$9 km /hr$
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$45 km /hr$
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$54 km /hr$

A gun fires a bullet of mass 50 g with a velocity of

$30 ms^{-1}$ . Because of this,the gun is pushed back with a velocity of

$1 ms^{-1}$ . The mass of the gun is.

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5.5 kg
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3.5 kg
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1.5 kg
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0.5 kg

Explanation

Given,

$m_b=50g$

$v_b=30m/s$

$v_g=1m/s$

$m_g=?$

By the conservation of linear momentum,

$Initial$

$momentum$ =

$Final$

$Momentum$

$0=m_bv_b-m_gv_g$ minus sign because veloicity of gun is opposite to velocity of bullet.

$m_bv_b=m_gv_g$

$\dfrac{50}{1000}\times 30=m_g\times 1$

$m_g=1.5 kg$

The correct option is C.

A bullet of mass 25 g is fired from a rifle of mass 2 kg. Imagine that no other force are acting on the system. If the bullet travels with a velocity of 2

${ ms }^{ -1 }$ what will be the velocity of the recoil of the rifle ?

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0.025 m/s
0%
0.35 m/s
0%
0.45 m/s
0%
0.50 m/s

Explanation

Given,

$m_b=25g=0.025kg$

$m_r=2kg$

$v_b=2m/s$

Applying conservation of linear momentum,

$m_r\times v=m_b\times v_b$

where v is the velocity of the recoil of the rifle

$v=\dfrac{m_bv_b}{m_r}$

$v=\dfrac{0.025\times 2}{2}$

$v=0.025m/s$

A gun of weight of

$10 \,kg$ fires a shot of

$0.5 \,g$ with a velocity

$230 \,m/s$ . Velocity of recoil gun is-

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$1.51 \,cm/sec$
0%
$1.15 \,cm/sec$
0%
$1.5 \,cm/sec$
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$1.10 \,cm/sec$

Explanation

Given,

$m_g=10kg=10000g$

$m_b=0.5g$

$v_b=230m/s$

The recoil velocity of gun is,

$V=\dfrac{m_bv_b}{m_g}$

$V=\dfrac{0.5\times 230}{10000}=1.15\times 10^{-2} m/s$

$V=1.15cm/s$

The correct option is B.

A simple pendulum hangs from the roof of a moving train. The string is inclined towards the rear of the train. What is the nature of the motion of the train?

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Accelerated
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Uniform
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Retarded
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None of above

State whether the following statement is true or false:
Unbalanced forces acting on a body change its shape.

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True
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False

Is the following statement is true or false.
A rocket can propel itself in a vacuum.

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True
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False

An athlete runs some distance before taking a long jump, because:

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he gains energy to take him through the long distance
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it helps to apply larger force
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by running action and reaction forces increase.
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by running he gives himself large inertia of motion.

The acceleration of a body is to be doubled from its initial value. By what factor is the acting force to be increased ?

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half
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two
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four
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one

Explanation

As we know,

$Force \propto acceleration.$

So, if acceleration is doubled, the force will also increase by a factor of two.

Write true or false for the following statements :
The inertia of an object depends upon its mass .

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True
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False

Explanation

Yes, the inertia of the system depends on its mass.

Inertia is the property to resist the change in motion. The more the mass of the object, the more is the resistance to the change in velocity.

$F = m \times a$

For same force, if mass is more, then acceleration is less and thus mass is the measure of inertia.

Write true or false for the following statements:
Force may or may not produce any motion in a body.

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True
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False

Write true or false for below statement:
A force increases the mass of the body when applied on it.

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True
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False

Select the correct alternative:
A force to be expressed correctly requires.

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Only the magnitude
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Only the direction
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Both the magnitude and direction
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None of the above

The property of inertia is more in :

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A car
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A truck
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A horse cart
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A toy car

Explanation

Answer:

$A\, truck$
The property of inertia is directly proportional to the mass of the object.

When a body is acted by a constant force, then which of the following quantities remain constant

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Velocity
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Acceleration
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Momentum
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None of these

Explanation

$F = ma$

For a given body if

$F =$ constant then

$a =$ constant.

Newton's second law says that when a constant force acts on a massive body, it causes it to accelerate, i.e., to change its velocity, at a constant rate. In the simplest case, a force applied to an object at rest causes it to accelerate in the direction of the force

A rocket is propelled by a gas which is initially at a temperature of

$4000 \ K$ . The temperature of the gas falls to

$1000 \ K$ as it leaves the exhaust nozzle. The gas which will acquire the largest momentum
while leaving the nozzle is

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Hydrogen
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Helium
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Nitrogen
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Argon

Explanation

$\text{Momentum}=\text{Mass}\times \text{Velocity}$

Thus, the heavier gas will acquire the largest momentum.

Argon is the heavier gas among the given options. So, it will acquire the largest momentum.

Unit of momentum is:

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Newton metre
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Newton kg/ metre
0%
Newton metre/ sec
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Newton sec

Explanation

According to Newton's second law

$Force$ =

$\frac{dP}{dT}$

$dP=Force\times {dT}$

So unit of

$dP$ (Momentum) is

$Newton \times sec$

The inertia of a body depends on:

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The centre of gravity of a body
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Mass of an object
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Acceleration due to gravity
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Size of an object

The relation between force (F), mass (m) and acceleration (a) is:

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$F = ma$
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$m = aF$
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$a = mF$
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$ma = \dfrac{1}{F}$

Explanation

$F = ma$

The inertia of a body depends on

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the velocity of the body
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acceleration of the body
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the shape of the body
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mass of the body

Which of the following vehicle has the least inertia?

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Bicycle
0%
Scooter
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Car
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Truck

Four alternatives are given to the following incomplete statement/question. Choose the right answer.
If the forces acting on an object are balanced then

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the object will be accelerated
0%
the object will be in motion
0%
the object will have zero acceleration,
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the object loses its shape

Explanation

From Newton's second law of motion -

$F=ma$ , where

$F$ is the net force acting on a body,

$m$ is the mass of the body, and

$a$ is the acceleration of the body.

If the forces acting on the object are balanced then

$F=0$ .

$\therefore a=0$ , as

$m$ can not be zero.

Hence, the body will not have any acceleration.

What is the momentum of an object of mass m, moving with a velocity

$V$ ?

0%
$(mV)^{2}$
0%
$mV^{2}$
0%
$\frac{1}{2}mV^{2}$
0%
$mV$

Explanation

Correct Answer: D

Explanation:

Momentum defined as the quantity of motion of a body. It is a vector quantity and measured by the product of mass and velocity of the body. So,

momentum = mass ×velocity

$=m×V=mV$

Hence Option (D) is the right answer.

CBSE Questions for Class 9 Physics Force And Laws Of Motion Quiz 15 - MCQExams.com

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Practice Class 9 Physics Quiz Questions and Answers

CBSE Questions for Class 9 Physics Force And Laws Of Motion Quiz 15 - MCQExams.com

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Practice Class 9 Physics Quiz Questions and Answers

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