Force And Pressure - Class 8 Physics - Extra Questions

Classify the following as contact or non. contact forces :

Contact force is the force which exists when the bodies are in contact.

Here frictional force exist when two surfaces touch each other and moving in opposite direction.

Tension exist when a body hangs from a point with the help of either string or thread or anything. The same with normal reaction where as gravitational force exist even though bodies are not in contact but it is a short range force and a bit weak force.

Electrostatic force is a wide range force and same with magnetic force.Hence a,b,c are contact forces  where as d,e,f are not contact forces.


Give examples of forces which change the shape of the objects.

Examples of Forces that changes the shape of the objects:
(i) Force of compression acting on the walls of an inflated ballon when it is pressed between our hands.
(ii) The force acting on the dough when we roll a chapati/roti.
(iii) The force acting on a spring or a rubber band when we stretch it.
(iv) Force acting on two moving vehicles when they collide.


Give examples of forces which pull the objects : 

Examples of forces that pulls objects:
(i) Force developed in the muscles of our hands when we do pull-ups.
(ii) Force developed in our muscles when we pull the rope during tug of war
(iii) Force developed by a towing vehicle when it tows away another vehicle.
(iv) Downward force applied to a rope to hoist a flag.


Give any two examples of forces in case of push.

Examples of force that pushes objects:
(i) Force developed by the muscles in our hands when we push an object or person away from us.
(ii) Force in the wind that pushes a sailboat.
(iii) Force applied by a cricket bat to a ball while batting


A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

The force due to hammering causes the change in shape of iron and iron can be moulded in the shape of the required tool.


Give two examples of situations in which applied force causes a change in the shape of an object.

Below are the examples of situations in which applied force causes a change in the shape of an object.
  • When we press the foam, its shape is changed.
  • When we stretch the rubber bend,then its shape is changed.
  • Making of bread from dough.


The pull or push acting on a body is commonly called force.
State true or false.

A force is a push or pull. 
Therefore:
The pull or push acting on a body is called force.


To draw water from a well, we have to _______(push, pull) the rope.

When we draw water from a well, we move the rope towards ourselves.
Therefore, To draw water from a well we have to pull the rope.


The force applied by the archer to stretch the bow is an example of _______(contact, non-contact) force.

The force applied by the archer is a muscular force. Muscular forces are contact forces. The archer has to touch the bow in order to pull it.
Therefore, it is an example of a contact force.


What is Force?

Force is a physical quantity which deals with the dynamics of the motion. It causes body to move or to come at rest.
    Force is a vector quantity which has both magnitude and direction.


To move a loaded trolley we have to pull or push it.
State true or false.

To move a loaded trolley, we have to apply a force on it. This force can be a push or a pull. 


In a tug of war game, if the two teams exert equal forces, none of the team wins. Why?

In a tug of war game, if the two teams exert equal forces none of the team wins since it maintains the tug in the equilibrium position or in the same state as the forces are balanced forces. As both teams apply equal forces in opposite directions, the net force on the string becomes zero and no thereby no team wins.


Give two examples of situations in which applied force causes a change in the shape of an object.

Two examples of forces that cause a change in the shape of an object are:
(i) Rubber ball changes its shape when pressed
ii) Inflated balloon changes its shape when pressed between our palms.

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Fill in the blanks in the following statements:
(a) To draw water from a well we have to __________ at the rope.
(b) A charged body __________ an uncharged body towards it.
(c) To move a loaded trolley we have to ______________ it.
(d) The north pole of a magnet _____________ the north pole of another magnet.

(a) To draw water from a well we have to apply a pulling force at the rope.
(b) A charged body attracts an uncharged body towards it.
(c) To move a loaded trolley we have to either push or pull it.
(d) The north pole of a magnet repels the north pole of another magnet.


Give any two effects of a force on a non-rigid body.

Two main effects of force on a non-rigid body are:
(i) It can change the state of rest or of uniform motion of the body.
(ii) It can change the size or shape of the body.


Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Forces acting on a plastic bucket containing water held above the ground level in our hand are muscular force and gravitational force. Both the forces are same in magnitude but in opposite directions and therefore there is no change in state of its motion.

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Give two examples each of situations in which you push or pull to change the state of motion of objects.

The forces of push and pull work in opposite directions. The primary distinction between pull and push force is the direction in which the item exerts force in relation to the applied force.
Two examples of push:
1. Dislodging a large block from its initial location by pushing it.
2. We push the door in order to open the door.
Two examples of pull:
1. A bucket of water being pulled from a well.
2. Playing tug of war while pulling ropes.



A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

When you heat the iron, it softens. A blacksmith strikes the hot iron with his hammer, exerting force. The shape of iron is altered as a result of the applied force.


Define force? Mention some various types of force.

A force is a push or pull upon an object resulting from the object's interaction with another object. 

Some examples of forces are :

Frictional Force

Tension Force

Normal Force

Gravitational Force

Electrical Force

Magnetic Force



What is the basic mechanism behind all forces?

The basic mechanism behind all force is noting but interaction between the bodies
* some kind of forces such as frictional force need physical interaction by means of actual physical contact
*some kind of forces such gravitational force do not require any physical contact they can act any disatnce form object.


It is easier to pull a roller than to push it, why? 

The motion of a body depends upon effective force applied to it including its weight. As effective weight in the case of pulling is less than in the pushing, less force is required in pulling. Hence, it is easier to pull than to push.
 


What are the effects of a force acting on an object?

Effects of Force. A force acting on an object causes the object to change its shape or size, to start moving, to stop moving, to accelerate or decelerate. When there's the interaction between two objects they exert a force on each other, these exerted forces are equal in size but opposite in direction.


Water does not run out of a dropper unless its rubber bulb is pressed. Why?

Water does not run out of a dropper unless its rubber bulb is pressed because of the difference in liquid pressure and atmospheric pressure. There are two forces acting on the water inside the dropper. One is the atmospheric pressure outside the dropper and the other is the liquid pressure inside the bulb. Water is held inside the dropper against atmospheric pressure. When the bulb is pressed, the liquid pressure increases. When it becomes greater than atmospheric pressure, the water will come out of the dropper.



How many objects must interact for a force to come into play?

An interaction of one object with another object results in a force between the two objects. Thus, at-least two objects must interact for a force to come into play. 


Explain why, the tip of a sewing needle is sharp.
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The tip of the sewing needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth, producing a large pressure sufficient to pierce the cloth being stitched.


Explain why, a wide steel belt is provided over the wheels of an army tank.
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A wide steel belt is provided over the wheels of an army tank so that they exert less pressure on the ground and do not sink into it.


Identify the agent exerting the force, the object on which the force is acting and the effect of force in the following situation:
Squeezing out the juice from an orange.

Agent exerting the force can be a machine (electric power) or human. The object on which the force is acting is the orange. The force is moving the orange and changing the shape of it.


Identify the agent exerting the force, the object on which the force is acting and the effect of force in the following situation:
A child jumping to a height.

Agent exerting the force is the child himself (muscle of the child). The object on which the force is acting is the child himself. The force on child is changing its position.


Differentiate between contact and non-contact force. Support your answer with suitable examples.

 Contact forces Non-contact forces 
 These are forces which need a physical touch on the object to act upon it. These are forces which do not need a physical contact to act on an object.
 These may or may not be natural forces. These are natural forces 
 They can not work from a distance. They can work from a distance as well.
 Its various types are applied force, normal force, tension frictional force its various types are magnetic force electrostatic force,gravitational force
 Examples player kiicking a ball game of tug of war rolling ball stops itself  horse pulling a cart etc, Examples: magnet attracting iron nails, mango falling from tree, charged plastic ruler attracts bits of dry - paper etc.


Identify the agent exerting the force, the object on which the force is acting and the effect of force in the following situations .
Taking out ointment from a medicinal tube.

Agent exerting the force is human fingers. The object on which the force is acting is the medicinal tube. The force is changing the shape of the tube and moving the oinment out of the tube.


Explain why, a nail has a pointed tip.
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A nail has a pointed tip so that when it is hammered, the force of the hammer is transferred to a very small area of the wood creating a large pressure that pushes the nail into the wood.
$$P = \dfrac{ F }{ A }$$
$$P$$: Pressure
$$F$$: Applied force
$$A$$: Cross-section area
Here we can clearly pressure is inversely proportional to the area of the object's surface.


Why do camels have large flat feet?
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Camels have large flat feet so that there is a greater area in contact with the sand which produces less pressure on the sand and the camels can move easily on the sand.


Classify the following situation into contact and noncontact forces.
Applying brake in a bicycle.

A force that requires physical contact between objects is known as a contact force. 

Applying brake in a bicycle causes the brakes to rub against the tyres of the bicycle, and hence is an example of contact force


Fill in the blanks:
_______ is the SI unit of force.

Newton.


Define force. Write its $$SI$$ unit.

A push or pull acting upon an object as a result of its interaction with another object is called force. Its $$SI$$ unit is newton $$(N)$$.


Classify the following situation into contact and noncontact forces.
The speed of a ball rolling on ground is reduced.

Contact forces are ones which require a physical contact between the objects, on which it is being acted upon. 

In the given case, the speed of a ball rolling on ground is reduced due to frictional force.

Frictional force is a contact force, as it is exerted by a surface against the motion of a body across its surface.

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Give one example in each of the following cases:
A force changes the size of a body

Example of force changing size of the body:
In a cycle pump. when the piston is pressed, the air is compressed to occupy less volume.


Tabulate the following as contact force and non contact force.
pushing a trolley 
falling coconut from coconut tree
3.magnet attract iron nail
Drowing water from the well
pushing the car
earth is moving around the sun

Contract force -$$1,4,5$$
non contract force -$$2,3,6$$


State whether the given situation relates to a contact force or a non-contact force.
$$\text{The earth is revolving around the sun.}$$

The earth revolves around the sun under the action of gravitational force on the earth by the sun.
Gravitational force is a non-contact force. Therefore, the given situation relates to a non-contact force.


The cutting edges of tools like knives, blades etc are provided with sharp edges. Explain why ? 

The pressure acting on a surface area $$A$$ by a force $$F$$ is given by 
$$P=\dfrac{F}{A}$$
Hence pressure acting on a small surface area is large as compared to that on a large surface area. Hence cutting of objects becomes easier with sharp edges(small surface area).


We do not feel the pressure of the atmosphere around us, because the pressure in our bodies is roughly the same as the atmospheric pressure outside. Therefore, both the forces acting on our bodies cancel out and we do not feel uncomfortable. Type 1 for true and 0 for false

We do not feel the pressure of the atmosphere around us, because the pressure in our bodies is roughly the same as the atmospheric pressure outside. Therefore, both the forces acting on our bodies cancel out and we do not feel uncomfortable.


The cutting instruments are made sharp because it decreases the area of contact thereby increasing the pressure. State whether true or false. 

We know pressure is given by:
$$Pressure(P)=\dfrac{Force(F)}{Area(A)}$$
So pressure is inversely proportional to area of contact.
Hence less area means more pressure.That's why cutting instruments are made sharp so area of contact is very less and pressure becomes very high which makes the cutting process easy.


A wide steel belt is provided over the wheels of an army tank to increase the area of contact and so the pressure of  the heavy weight of the tank gets reduced. This prevents the tank from sinking into the ground. State whether true or false.

We know 
$$Pressure=\dfrac{Force}{Area}$$
So pressure is inversely proportional to area of contact.
So wide steel belt ensures weight of tank falls on larger area of ground hence pressure exerted is less hence tank does not sink into ground.
So So above statement is true.


We hammer the broader side of a nail and not its sharp end to put it in wood. Why?

By doing so, the force will be made to act over the least area and so it would exert maximum pressure on the nail. As a result, the sharp end of the nail would more easily pierce into the wood.


Wooden or concrete sleepers are kept below the railway line to increase the area of contact of the train. As a result, there is less pressure of the train on the ground preventing it from sinking. State whether true or false. 

We know 
$$Pressure=\dfrac{Force}{Area}$$
So pressure is inversely proportional to area of contact.
So wooden or concrete sleepers have large contact area with ground hence less pressure is exerted on ground and train does not sink.So above statement is true.


$$a$$. Squeezing a piece of lemon between the fingers to extract its juice.
$$b$$. Taking out paste from a toothpaste tube.
$$c$$. A load suspended from a spring while Its other end is on a hook fixed to a wall.
$$d$$. An athlete making a high jump to clear the bar at a certain height.

Below is the explanation.

a. The fingers are the agents, lemon is the object. The effect of force is the lemon juice being expelled by squeezing.
b. The hand is the agent, toothpaste tube is object and the coming out of paste from toothpaste tube is the effect of force.
c. Suspended load is agent, spring is the object, the effect of force can be seen in the form of elongation of spring on suspension of load.
d. Athlete is the agent, bar is the object. The force can be seen in the form of jump


Why do mountaineers usually suffer from nose-bleeding at high altitudes?

Air pressure goes on decreasing as we go higher up, so at high altitudes, the atmospheric pressure becomes less than the pressure exerted by the blood inside the body. This causes some of the blood vessels burst and the nose starts bleeding.


A german scientist joined two hollow metallic hemispheres and created a vacuum inside them by pumping the whole air out. Then he employed eight horses on each hemisphere to pull them apart but the horses failed to do so. Give reason for this.
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There being no air inside, there was no opposition to the outside atmospheric pressure acting over the whole surface of hemispheres with the result that this outside pressure pressed the hemispheres very very hard and did not allow them to be separated.


 Match the following :

A: In opening a drawer, we pull it out towards us. => 3. Pull
B: Electrostatic force acts at a distance, it is a field force. => 5. Non-contact force
C: In closing a drawer, we push it away from us. => 1. Push
D: Muscular forces are forces applied by the muscles in our limbs when push or pull something => 2. Contact forces
E: A force is defined as push or a pull. => 4. Push or pull
F: in liquids, the pressure at a point is equal to $$\rho gh$$ where $$h$$ is the depth of the point from the liquid surface. => 6. Depth


Two boys exert forces of $$40 N$$ and $$60 N$$ in opposite directions on an object. Then calculate the resultant force.

Resultant force is nothing but the total force acting on a body including their directions. If the object is at rest or it moves with a same velocity, the resultant force should be zero. All forces are acting on same direction, resultant force should be the sum of all the forces.
In this case, two boys exert forces of 40N and 60N in opposite directions on an object. This means that one force goes with a plus sign and the other force goes with the minus sign. 
That is, $${ F }_{ Resultant }={ F }_{ 1 }-{ F }_{ 2 }=60N-40N=20N$$.
Hence, a force of 20 N acts in the direction of greater force.


Mention two cases in which it is useful to decrease the pressure exerted by a force and two cases in which it is useful to increase the pressure exerted by a force.

Useful to decrease pressure
(1) Snow boots have large surface area to minimize pressure.
(2) When you hang bag in two shoulder it feels lighter than when you having it in one hand.
Useful to increase pressure
(1) Nails have pointed ends to increase pressure by minimizing surface area.
(2) Knifes are sharpened to increase pressure or apply maximum pressure with less force.


How is a nail able to pierce the surface of wood ?

Since, $$Pressure(P)=\dfrac{Force(F)}{Area(A)}$$

In a nail, the sharp end is for exerting maximum pressure by minimizing the effective surface area. Hence increase in pressure makes the nail pierce the surface of wood.
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An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms:
Muscular, contact, non-contact, gravity, friction, shape, attraction
(a) To stretch the bow, the archer applies a force that causes a change in its _____________ .
(b) The force applied by the archer to stretch the bow is an example of ______________ force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a ______________ force.
(d) While the arrow moves towards its target, the forces acting on it are due to ___________ and that due to ___________ of air.

(a) To stretch the bow, the archer applies a force that causes a change in its shape.

(b) The force applied by the archer to stretch the bow is an example of muscular force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force..

(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.


State the effect produced by a force in the following case :

A spring is compressed.

Effect of force:

When a spring is compressed the size of the spring changes i.e. there is a decrease in the length of spring due to compression.




Types of forces a fluid exerts are ............, ............... and  ................

downward ,upward and lateral.
A fluid can exert downward force on base of container in which it is stored.And it can exert upward force of buoyancy on an object submerged in it. And fluid can also exert lateral force on side walls of a container.


Sideways pressure exerted by a liquid is called its lateral pressure.
True =1; False=0

Sideways pressure exerted by liquid because of its tendency to flow is called lateral pressure. When a bottle having water is pierced on the sides we can see water coming out speedily from the holes. This is because liquid exerts lateral pressure on the walls of the container.
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State true or false.
Animals like camels walk easily in the desert as broad feet exert great pressure on the sandy ground.

Camels can walk easily in the desert because of their wide feet. Their wide feet allow the weight of their body to act on a larger surface of land reducing the pressure exerted on the land by the camel. As we know, pressure exerted and area are inversely proportional.



Liquids exert pressure on the wall of the container because they don't have any preferred direction.
Mark 1 if true else mark 0

At a particular level in a fluid, pressure is the same in all the directions. So it is true that liquid exerts pressure on the wall of the container.


It is easier to fix a sharp nail than a blunt nail for the same force applied, this is because of difference in their _______

Pressure is defined as force per unit area ,$$P=\dfrac{F}{A}$$
The sharp nail has smaller area than the blunt nail, hence for the same force the pressure for sharp nail will be higher.  


A force is applied on:
(i) a non-rigid body
(ii) a rigid body.
How does the effect of the force differ in the above two cases?

A force, when applied on a rigid object, does not change the inter-spacing between its constituent particles and therefore it does not change the dimensions of the object, but causes motion in it. On the other hand, a force, when applied on a non-rigid object, changes the inter-spacing between its constituent particles and therefore causing a change in its dimensions.


The sports boot for soccer and hockey have studs on their soles. Why?

They need better grip with ground. Studs reduces the area for which the weight is acting on ground . Due to smaller area, it produces high pressure which gives a better grip.


Why it may be difficult to stop bleeding from a cut in the body at higher altitudes?

The atmospheric pressure is low at high altitudes. Due to the greater pressure difference in blood pressure and the atmospheric pressure, the blood keeps flowing out of the body and hence it is difficult to stop bleeding from a cut in the body.


If 50 N force is applied on a liquid and it experiences $$25 N/m^2$$ pressure. Find out the area on which the force is applied?

Pressure = $$\dfrac { Force }{ Area } $$
Given : Pressure = 25 N/$${ m }^{ 2 }$$
             Force = 50 N
Hence Area = $$\dfrac { Force }{ Pressure } =\dfrac { 50 }{ 25 } { m }^{ 2 }=2{ m }^{ 2 }$$


When we suck using a straw, the liquid travels up into it. Explain why?

When we suck using a straw, we create a pressure difference at the two ends of the straw. Which drives the liquid up the straw. Liquid travels from high pressure to low pressure. Upward low pressure created and hence liquid rises in straw.


Swetha is wearing a sharp edged high heeled footwear. Madhu is wearing a flat footwear. If both the girls are having same weight and both are stepping over your foot, whose footwear will cause more pain to your foot? Why?

Since swetha footwear is sharp edged, hence area of application of force is less. Due to this pressure exerted by swetha footwear is more than that by Madhu. Hence Swetha footwear will cause more pain.


Arasu went to an exhibition. There he saw a magician lying on a bed of nails. To his wonder, the magician was not hurt at all. Help Arasu to understand the phenomenon.
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By increasing the number of nails & application area by sleeping over bed of nails the pressure exerted is very less which help the magician to be safe and do not get hert.


List out the following actions on the basis of contact force and non-contact force.
a) Lifting a chair
b) The falling a coconut from the tree
c) Friction between the road and the tyre of a car
d) A comb attracts bits of paper
e) Attraction between two magnets

Contact force: Force or effort which transfers to a body due to physical contact is called Contact force. ex- Friction force etc
Non-contact force: Force or effort which can transfer without any physical contact is called Non-contact type of force. ex- electrostatic force etc.

a) Lifting Chair:
    To lift the chair contact is required between hand and chair. hence it is contact type of force.

b) The falling of a coconut from the tree.
    On coconut non-contact force is exerted by the gravitational field.

c) Fraction between road and car.
    Friction is an example of contact types of force.

d) A comb attracts bits of paper.
   It is an example of electrostatic force exerted by the electric field. It's a non-contact force.

e) Attraction between two magnets.
  It is a non-contact force act between two magnets as a magnetic force.


Aswin and Anwar were playing with four bricks each. Aswin arranged his four bricks as shown in figure A. Anwar arranged his bricks as shown in figure B, in order to be a taller one.
Now let us complete the following sentences by choosing the right option below
(equal to, less than, more than)
a) The force of A on the ground is __________ the force of B on the ground.
b) The area that A occupies is ________ B on the ground.
c) The pressure exerted by A is _______ the pressure exerted by B.
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Given,
Aswin and Anwar were playing with four bricks each.
We know,
Force, $$F=m\times a$$
$$m=$$ mass of four bricks
$$a=g=$$ acceleration due to gravity
a) The force of $$A$$ on the ground is equal to the force of $$B$$ on the ground.

Anwar arranged the bricks on the edge.
b) The area that $$A$$ occupies is greater than $$B$$ on the ground.

We know,
$$Pressure=\dfrac{Force}{Area}$$

For equal forces, $$Pressure\propto \dfrac{1}{Area}$$
Therefore,
c) The pressure exerted by $$A$$ is lesser than the pressure exerted by $$B$$.


Why are railway tracks laid on large size concrete sleepers ?


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A solid weights $$500\ N$$. Calculate the pressure extended by solid on surface if the area of contact is $$100\ cm^2$$.

We know that
$$\text{Pressure} = \dfrac{\text{Thrust}}{\text{area}}$$

Given,
$$\text{Thrust} = 500\ N$$ and 
$$\text{area} = 100\ {cm}^{2}$$

Thus Pressure $$=\dfrac { 500 }{ 100\times { 10 }^{ -4 } }$$ =$$50kPa$$


Why do lorries carry heavy loads have more tyres with spread tyres?

Lories carry heavy loads have more tyres with spread tyres because the carrying heavy loads use more tyres to reduce the pressure on its tyres. As the surface area increases the load is spread over a larger area and hence reducing the pressure. This reduction in pressure allows the lories to move batter and also protects the tyres from excessive wear and tear.

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



Define force. What changes can be produced by force?

Force is an external agent (push or pull) acting on an object which changes or tends to change the state of rest or motion of an object. It is a vector quantity.
The SI unit of force is Newton $$(N)$$.
Force can produce the following changes when acts on an object:
i. the state of rest or motion of an object
ii. the shape of an object
iii. the size of an object
iv. the direction of motion
v. the speed of motion


How can you demonstrate the process of atmospheric pressure experimentally?

To demonstrate the process of atmospheric pressure experimentally, the following procedure can be followed :
1. A metallic can is taken and filled with water.
2. Its lid is removed and the can is heated.
3. The water starts boiling and steam starts coming out from the mouth of the can.
4. The steam forces out most of the air from the can.
5. The can is closed with an air-tight lid.
6. Cold water is added to the can and it starts crashing.

Explanation: When the can is heated and covered with a lid, the can becomes filled with hot air (steam). On pouring cold water, it condenses to a liquid and thus a partial vacuum is created inside the can. Thus, the atmospheric pressure becomes more than the internal pressure due to which the can is crushed. 


Why is pulling easier than pushing ? Explain with the help of diagram.


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Why is it easy to walk on sand with flat shoes, then with high heel shoes?

As $$Pressure=\dfrac{Force}{Area}$$
$$\Rightarrow$$ Pressure$$\propto\dfrac{1}{Area}$$
As high heel shoes have smaller contact area, hence it exerts high pressure and shoes go inside the sand. So it is difficult to walk on the sand with high heels. But for flat shoes, the contact area is more and hence less pressure is acting. Therefore, the shoes do not go inside the sand. 


When we stand on loose sand, our feet go deep into the sand. But when we lie down on the sand our body does not go that deep into the sand. But when we lie down on the sand our body does not go that deep in the  sand. Why?

As pressure$$=\dfrac{Force}{Area}$$
$$\Rightarrow$$ Pressure $$\propto\dfrac{1}{Area}$$
If we stand, we apply more pressure than when we are lying down due to less area of contact(Force$$=$$weight$$=$$constant).

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Why is it difficult to hold a school bag having a strap made of a thin and strong string?

We know,
$$Pressure=\dfrac{Force}{Area}$$

It is difficult to hold a school bag having a strap made of thin and strong sting$$,$$ because the thin string has a very little contacting area in case of a school bag having a strap made of thin and strong string which increases pressure which is uncomfortable to carry the school bag$$.$$  


A perpendicular  force of 50 N acting on a surface generates a pressure of 250 pa. calculate the area of cross section of the surface on which  pressure is acting .

Given,
Pressure $$P=250\ Pa\ =250\ Nm^{-2}$$
Force applied $$F=50\ N$$
As pressure $$=\dfrac{Force}{Area}$$

        $$\Rightarrow$$ Area $$=\dfrac{Force}{Pressure}$$

         $$\Rightarrow$$ Area $$=\dfrac{50}{250}=\dfrac{1}{5}m^2=0.2m^2$$.

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What are the various effects of force? State one example of each.

Various effects of force :  
$$1.$$ A force can move a stationary object. Example: Pulling a suitcase on its wheels.  
$$2.$$ A force can stop a moving object. Example: A ball rolling on a rough surface comes to rest after some time due to the force of friction. 
$$3.$$ A force can change the speed of a moving object. Example: We can increase the speed of a moving swing, we apply a force (push) in the direction of motion to increase its speed.  
$$4.$$ A force can change the direction of a moving object. Example: A moving bicycle changes its direction when a force is applied to steer its handle. 
$$5.$$ A force can change the shape and size of an object. Example: When we make a chapatti, a small ball of dough is changed to a flat, circular shape of a much bigger size by applying a force with the help of a rolling pin.


On what factors does the effect of thrust on a surface depend? 

The effect of thrust depends on the area of the surface on which it acts . smaller the area of the surface on which a thrust acts , larger is its effect. but the effect of a thrust is less on a larger area.


What effect can a force have on a moving body?

When a force is applied on a moving body, it 

1.) can stop a moving body :

  • When a force is applied on a moving body in a direction opposite to the direction of motion of the body, it can be made to stop. 

  • For example,  a moving  bicycle, bus or train is stopped by applying the brakes. The brakes act against the direction of motion of bicycle, bus or train. Thus, the brakes provide the stopping force. 


2.) can make a moving body move faster : 

  • When a force is applied on a moving body in the direction in which it is moving, the body begins to move faster.

  •  For example, the speed of a bicycle increases, when more force is applied on the pedal by the cyclist.


3.)  can slow down a moving body : 

  • When a force is applied on a moving body in a direction opposite to the direction of motion of the body, the body slows down or stops. 

  • For example, the speed of a moving vehicle is slowed down by applying the brakes. The brakes provide a force in a direction opposite to the direction of motion. 


4.) can change the direction of motion of the body : 

  • If a force is applied on a body in a direction other than its direction of motion, the direction of motion of the body changes.

  • For example, a player kicks a moving football to change its direction of motion, a player applies force with a hockey stick to change the direction of motion of the ball.



A force of $$25 \ N$$ acts on an area of 25 $${ cm }^{ 2 }.$$ Calculate the pressure produced. 

Given:
Force $$F=25 \ N$$
Area $$A=25 \ cm^2=25 \times \dfrac{1}{100} \ m \times \dfrac{1}{100} \ m=\dfrac{25}{10^4} \ m^2$$

Pressure $$P =\dfrac{Force \ F }{Area \ A }$$

$$P=\dfrac{F}{A}=\dfrac{25}{25 \times 10^{-4}}=10^4 \ N/m^2$$

Since $$1 \ N/m^2= 1 \ Pa$$

$$\therefore$$ Pressure $$P=10^4 Pa$$


Explain why, school bags are provided with wide straps to carry them.

School bags have wide straps so that their weight may spread over a large area of shoulder producing less pressure on the shoulder.


When is the pressure on the ground more-when a man is walking or when a man is standing? Explain.

When a man is walking, then at one time only one foot is on the ground. Due to this, the force of weight of man falls on a smaller area of the ground and produces more pressure on the ground. On the other hand, when the man is standing, then both his feet are on the ground. Due to this, the weight of the man falls on a larger area of the ground and produces lesser pressure on the ground.


What force acting on an area of $$0.5 m^2$$ will produce a pressure of 500 Pa?

$$Pressure = \frac{Force}{Area}$$
$$Force =  Area \times pressure$$
$$ = 0.5 \times 500$$
$$ = 250 N$$


State the various effects of force.

Various effects of force are
i) A force can move a stationary body.
ii) A force can stop a moving body.
iii) A force can change the speed of a moving body.
iv) A force can change the direction of a moving body.
v) A force can change the shape and size of body.


Give a example each where:
a) a force moves a stationary body.
b) a force stops a moving body.
c) a force changes the speed of a moving body.
d) a force changes the direction of a moving body
e) a force changes the shape (and size) of a body.

a) kicking a stationary football
b) Applying breaks to a moving bicycle.
c) Pressing an accelerator to speed up a moving car.
d) A moving cricket ball hit by a bat.
e) Flattening of dough by a rolling pin to make chapatis.


Explain why, snow shoes stop you from sinking into soft snow.

Snow shoes stop us from sinking into soft snow because due to large area of snow shoes, our weight is spread over a large area of the snow producing small pressure.


Define the term 'force'.

Force is an influence which tends to set a stationary body in motion or stop a moving body, or which tends to change the speed and direction of a moving body, or which tends to change the shape and size of body.


Why cutting Instruments are sharpened?

The cutting instruments apply high pressure on a body to cut it as per requirement. 

Since Pressure $$P = \dfrac{Force}{Area}$$

To apply high pressure on a body, the area of contact should be minimum. 

Thus, the cutting Instruments are sharpened to decrease the area of contact and increase the pressure applied by them. 

This results In smoother and better cutting with lesser force. 

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Define force. List any two effects of force.

Force is generally described as a push or a pull. More specifically, force is any interaction which when unopposed, changes the motion of the object subject to that force. Mathematically, force is the product of mass and acceleration or the rate of change in momentum.
$$ F = ma $$ 
Effects of force:
1. Force can change the direction of motion of the object. e.g. A footballer uses a force to change the direction of motion of a ball.
2. Force can change the speed of any moving object. e.g. While catching a ball hit by a batsman, the fielder changes the speed of the ball to zero by applying force. 


Explain why, when a person stands on a cushion, the depression is much than when he lies down on it.

When a person stands on a cushion then only his two feet (having small area) are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure causing a big depression in the cushion. On the other hand, when the same person lies down on the cushion, then his whole body (having large area) is in contact with the cushion. Here, his weight falls on a much larger area of the cushion producing much smaller pressure and very little depression in the cushion.


When is the pressure on the ground more when a man is (i) lying (ii) standing on the ground?

Pressure is given by
$$P=\frac{F}{A}$$  $$F$$ is force $$A$$ is area
Hence the pressure applied on an object is inversely proportional to the area of contact. 
When the man Is lying on the ground. the area of contact Is much more than that when standing. 

Thus, the pressure Is more when the man Is standing on the ground. 


Explain why, buildings and dams have wide foundations.

The foundations of buildings and dams are laid on a large area of ground so that the weight of the building or dam produces less pressure on the ground and they may not sink into the ground.


Use your ideas about pressure to explain why it is easier to walk on soft sand if you have flat shoes rather than shoes with sharp heels. 

Flat shoes have greater area in contact with the soft sand as compared to heels. Due to this, there is less pressure on soft sand because of which they do not sink much in the sand and it is easy to walk on it.


Name the term used for the push or pull?

Force.
A force is any influence that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity, i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity.


An elephant weighing 40,000 N stands on one foot of area $$1000 \ cm^2$$ whereas a girl weighing 400 N is standing on one 'stiletto' heel of area $$1 \ cm^2$$.
(a) Which of the two, elephant or girl, exerts a larger force on the ground and by how much?
(b) What pressure is exerted on the ground by the elephant standing on one foot?
(c) What pressure is exerted on the ground by the girl standing on one heel?
(d) Which of the two exerts larger pressure on the ground : elephant or girl?
(e) What is the ratio of pressure exerted by the girl to the pressure exerted by the elephant?
1763673_1669280146674d5ea05301aa6416c349.png

Weight of elephant=40000N 
Area of one foot  = 1000 $$cm^2 = 1000 \times 10^{-4}m^2$$
Weight of girl=400N
Area of heel of girl  = 1 $$cm^2 = 1 \times 10^{-4}m^2$$
(a) Elephant has a larger weight of 40000N, therefore, elephant exerts a larger force on the ground. Elephant exerts a larger force on the ground by 40000N  - 400 N = 39600 N.
(b) Weight of elephant = 40000N
Area of one foot = 1000 $$cm^2 = 1000 \times 10^{-4} m^2$$
$$Pressure = \frac{Force}{Area}$$
$$Pressure  = \frac{40000}{1000 \times 10^{-4}} = 400000 N/m^2$$
(c) Weight of the girl = 400N
Area of heel of girl =1 $$cm^2 = 1 \times 10^{-4}m^2$$
$$Pressure = \frac{Force}{Area}$$
$$Pressure  = \frac{400}{1 \times 10^{-4}} = 4000000 N/m^2$$
(d) Girl exerts a larger pressure on the ground.
(e) Ratio = $$\frac{Pressure \ exerted \ by \ the \ girl}{Pressure \ exerted \ by \ the \ elephant} = \frac{4000000}{400000} = \frac{10}{1}$$
The pressure exerted by girl is 10 times greater than that exerted by the elephant.


Where do we apply a force while walking? 

We apply pressure on the ground and the reaction the force exerted by the ground helps us to move forward. 


Answer the following in a word or two or in a sentence.
Give an example in which a force can change the direction of an object.

In a cricket match, when a moving cricket ball is hit by a bat ,then the direction  of cricket ball changes and it goes in another direction. The force exerted by bat changes the direction of moving cricket ball.


Explain the meaning of the term force.

Force : Force is a physical cause that changes or may tend to change the state of rest or the state of motion of an object. The S.I. unit of force is Newton.


Which force is responsible for supporting the weight of a book placed on a table?

According to Newton's third law of motion, to every action, there is an equal and opposite reaction and they act on two different bodies. The book while placed on the top of the table exerts a force on the table (book's weight due to gravity), and the table reacts with an equal and opposite force.


What effect can a force have on a stationary body?

We know that force when applied to a body can change its state from rest to motion or from motion to rest.

So, when a force is applied to a stationary body,

  • The body starts moving

  • If the body is not free to move, it deforms due to the force applied.


For example,

  •  a car originally at rest when pushed, begins to move

  • A ball lying on the ground moves, when it is kicked.



Name the type of force which acts in the following cases:
A boy opens the door.

Muscular force.


Name the two kinds of forces in nature.

Two kinds of forces in nature are:
1. Contact forces
2. Non-contact forces.


Name the type of force which acts in the following cases:
A horse moves a cart.

 Muscular force


Explanation:


When a horse moves a cart, the muscular force of the horse is used to pull the cart.

i.e Horse applies the force using  muscles of the body




Name the type of force which acts in the following cases:
A man rows a boat.

 Muscular force.


Explanation


When a man rows a boat, the man uses his hands for rowing. So, he uses muscular force through his hands for rowing.




Give one example each of a force as
a push

  • Movement of a body is obtained as a result of a push or a pull. This push or pull is termed as force.

  • In push : body moves away from the person applying force

  • Example of force as push :To move a stopped trolley, we push the trolley forward. 





Describe an experiment to show that a liquid exerts pressure at the bottom of the container in which its is kept.

A LIQUID EXERTS PRESSURE AT THE BOTTOM OF ITS CONTAINER :
take a balloon and tie it at the lower end of a glass tube and hold vertically as in fig (a) pour some water in the tube , balloon bulges our as in
(b) Because water column exerts pressure $$ ( \therefore $$ liquid has weight at its bottom . force on balloon is equal to the weight of Thrust water column which is called the thrust P = Thrust  / Area 
This shows that a liquid exerts pressure at the bottom of the container in which it is kept.

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Which fact about liquid pressure does the diagram in fig. illustrate.
1802569_bf8d614d71654818a4c4f0e2a0d37833.png

As we can see, In fig-(a) Height of liquid in the tube is small at the same time size of the balloon is small. Whereas in fig-(b) Height of water in the tube is more and size of the balloon is big. 
As we know, the Size of a balloon depends on the pressure inside it. And its size increases with increase in pressure inside it.  
Now, it is clear that the balloon in fig-(b) have high pressure. And this high pressure is developed due to the height of water level in the tube. 
From above demonstration, we can say that, Water pressure increase with height of its level.


A dam has broader walls at the bottom than at the top. Give a reason.

The pressure of a liquid at a depth $$d$$ is $$P =\rho g d$$, where $$\rho$$ is the density of the liquid and $$g$$ is the gravitational acceleration.
The bottom of the dam is at a depth $$d$$. Therefore the pressure on the bottom of the dam is more than the pressure on the top of the dam.
To withstand the greater pressure of water which increases with an increase in depth the bottom wall of a dam is broader.


Name two factors on which the pressure on a surface depends.

FACTORS AFFECTING THE PRESSURE.
(i) The surface are $$ (A) $$ on which thrust acts. More area , lesser is pressure.
(ii) The magnitude of thrust $$ (F) $$  acting on the surface 
$$ \because P = \frac {Thrust}{Area } = \frac {F}{A} $$ More thrust , more is pressure.


Answer the following in detail.
What are the various effects of force? State one example of each.

Various effects of force:
1. A force can move a stationary object. Example : Pulling a suitcase on its wheels.
2. A force can stop a moving object. Example : A ball rolling on rough surface comes to rest after some time due to force of friction.
3. A force can change the speed of a moving object. Example : We can increase the speed of  moving swing, we apply a force (push) in the direction of motion to increase its speed.
4. A force can change the direction of a moving object. Example : A moving bicycle changes its direction when a force is applied to steer its handle.
5. A force can change the shape and size of an object. Example : When we make a chapatti, a small ball of dough is changed to a flat, circular shape of a much bigger size by applying a force with the help of a rolling pin.


When does a man exert more pressure on the floor: while standing or while lying on the ground?

$$Pressure=Force/Area$$
So Pressure is inversely dependent on area of contact.
Since contact area is less in standing case as compared to when lying on the ground so man exert more  pressure while standing.


A sharp pin works better than a blunt pin. explain the reason.

A sharp tip is provided at the end of a pin so that pressure $$ (FORCE /AREA) $$ exerted by it is maximum when it is pressed by a given force.


List all the forces acting on a boy sitting on a chair with the help of a suitable diagram.

The forces acting on a boy sitting on a chair are -
1. Gravitational force (non-contact) acting downwards - $$F_{grav}$$.
2. Normal contact force by the chair acting upwards - $$(F_{norm})$$
Both the forces are opposite and equal and hence they balance each other.

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What do you mean by atmospheric pressure ?

The gaseous layer around the surface of the earth is called the atmosphere. The components of the atmosphere apply a thrust on the surface of the earth.
$$\text{ATMOSPHERIC PRESSURE:}$$ The thrust due to atmospheric air on unit area is called the ATMOSPHERIC PRESSURE
1697342_1802577_ans_97b04ffd489c4ea3bc43f2fa7fd53734.png


Why is the bottom part of foundation of building made wider ?

$$ P = F/A $$ 

$$ P \propto 1/A $$ 
pressure is inversely proportional to surface area.
Greater the area , lesser is the pressure.
In order that building should exert less pressure . the foundation of building should have more area or be wider.So that building should not sink into the earth.


Give one example in case where force acts from a distance.

The forces which act on bodies when they are not in physical contact, are called non-contact forces. 
The gravitational force on an object applied by the earth is an example of non-contact force.


We do not feel uneasy even under the enormous atmospheric pressure . Give a reason.

Blood in our body exerts $$ BLOOD  \, PRESSURE $$ which is slightly more than atmospheric pressure and we do not feel uneasy because we are accustomed to comparatively larger pressure.


What are contact forces? Give two examples.

$$\underline {\textbf{Contact Forces:}}$$
The forces which act on bodies when they are in $$physical \ contact$$ are called contact forces. 

$$\underline {\textbf{Example:}}$$
$$\bullet$$ Any physically applied $$push/pull$$ - pulling a string, pushing a chair
$$\bullet$$ $$Normal \ reaction$$ -  Normal reaction is applied on a body by any surface at a perpendicular direction to the surface.
$$\bullet$$ $$Friction$$ - Friction force acts when two bodies in contact are in motion with respect to each other. Friction always opposes the relative motion between two bodies.


Classify the following amongst contact and non-contact forces.
(a) Frictional force
(b) normal reaction force.
(c) force of tension in a string
(d) gravitation force
(e) electrostatic force
(f) magnetic force

The forces which act on bodies when they are in physical contact. are called contact forces. For e.g. frictional force and force exerted on two bodies during a collision.
The forces experienced by bodies even without being physically touched are called the non-contact forces. For e.g. Gravitational force and Electrostatic force.
Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string
Non-contact force: (d) gravitational force (e) electric force (f) magnetic force


The base of a container measures $$ 15 cm  \times  20 cm $$ it is placed on a table top . if the weight of the container is $$ 60 N $$, what is pressure exerted by the container on the table top ?

Area of the base of container
$$ A  = 15 cm  \times 20 cm $$
$$ = \dfrac { 15}{100} m \times  \dfrac {20}{100} m $$
$$ = \dfrac {300}{10000}m^2 $$
Force  = weight = $$ 60 N $$
Pressure exerted = $$ \dfrac {F}{A} = \dfrac { 60}{ \dfrac {300}{10000} }  $$
$$ P  = \dfrac { 60 \times 10000}{300 } = \dfrac {10000}{5} = 2000 Pa $$


Describe a simple experiment to illustrate that air exerts pressure.

EXPERIMENT :  Take a tin-can having air tight cap ( screw cap) . Remove cap and boil Some water in it , so that steam comes out and in this way air from inside goes out. 

While boiling replace the cap and allow the can to cool, by pouring some cold water on it. Vapours inside condense and form water, creating vacuum above them. After some time, we see that the can crumbles due to air pressure from outside. This proves that air exerts pressure.

1697314_1802588_ans_2e76a1090bfb4f26bfa677e53b73939f.png


What are non-contact forces? Give two examples.

The forces experienced by bodies even without being physically touched are called the non-contact forces. For e.g. Gravitational force and Electrostatic force.


State one factor on which the magnitude of a non contact force depends. How does it depend on the factor stated by you?

The magnitude of a non-contact force depends on the distance of separation of two bodies. Magnitude of force decreases as the distance increases.

$$F=\dfrac{1}{r}$$


Give one example where the force is of contact.

The forces which act on bodies when they are in physical contact. are called contact forces. For e.g. frictional force and force exerted on two bodies during a collision.
Force exerted on two bodies during a collision, is a contact force


Give one example in each of the following cases where a force:
Stops a moving body

Listed below are the examples for the following cases:
In the game of cricket, a fielder, by applying force with his hands stop a moving ball. 


State the effects of a force applied on:
(a) A lump of dough on a plate 
(b) A ball at rest

a) When a force is applied to a lump of dough on a plate, the shape of the dough changes.
b) When a force is applied on a ball at rest it changes its state of rest and the ball will start moving.
Force acting on an object may cause a change in its state of motion or a change in its shape.


Explain giving two examples each of :
Contact forces, 

$$Contact\, forces -$$ These are the forces that act upon bodies when they are in physical contact are known as contact forces. Example - Force exerted on two bodies while the process of collision, frictional force.


Give one example of the following :
A force moves a stationary body.

When force is applied on a stationary object, the object tends to move in the direction in which force is applied. For example, applying force on a stationary tennis ball while hitting it with a bat will make it move in the direction of applied force by the bat.


Give one example where A force stops a moving body.

Example of force stoping a body:
A fielder on the ground stops a moving ball by applying a force with his hands.


Give one example  where:
The force is  contact-force.

A contact force is any force that requires contact to occur. Contact forces are  responsible for most visible interactions between macroscopic collections of matter. Pushing a car up a hill or kicking a ball across a room are some of the everyday examples where contact forces are at work.


Give one example in each of the following cases:
A force changes the shape of a body

Example of force changing shape of body:
On pressing a piece of rubber, its shape changes.


State the condition when a force produces:
rotational motion.

Force can produce both translation and rotational motion
When the body is pivoted at a point. it produces rotational motion.


Give one example in each case where:
Force is at a distance.

A force that acts on an object without being in contact (at a distance)  is known as a non-contact force. A magnet can exert a force on another magnet without being in contact with it. The force exerted by a magnet is an example of a non-contact force.


Explain giving two examples each of :
non-contact forces

$$Non - contact\, forces -$$ These are the forces that are experienced by the bodies without any physical contact is known as non - contact force. Example - Electrostatic force and gravitational force.


Explain the following:
A balloon collapses when air is removed from it.

When air leaves a balloon, the pressure inside is far lesser than the pressure outside the balloons. Therefore, the balloon collapses when air is removed from it.


Give one example where a force changes the size of a body.

Force can change the size of the body. 
Ex: Take a rubber band and pull it through its end. You can see that length of the rubber band increase and when this force removed then it may or may not regain their shape. Hence this activity prove that force can change the size of the object.


Is force needed to keep a moving body in motion?

No, force is not required to be a moving body unless the direction or the speed is require to be changed.


Explain the following.
Sleepers are laid below the rails .

Sleepers are usually laid below the rails to spread the weight of the moving train over to a larger area. This is done to increase the area so that the pressure exerted by the rail gets decreased. Thus providing safety to the movement of the trains on the rail tracks.


Give one example where a force changes the shape of the body.

Rubber bands change their shape by applying force. If the force removes, it may or may not regain its original shape.
Hence force can change the shape of objects.


What do you understand by atmospheric pressure ?

The pressure exerted by the air is known as atmospheric pressure. Pressure is defined as force per unit area. Consider a very long cylinder of unit cross-section area, then the weight of the air in this cylinder is the atmospheric pressure.


A ball moving on a table top eventually stops. Explain the reason.

A ball moving on a table top is restricted by the force of friction between the ball and the table top as it opposes the motion. 

Friction is the force that resists motion when the surface of one object comes in contact with the surface of another.
So gradually the ball stops as friction force opposes the motion.




Define one newton.

One newton is defined as that force which acting on a body of mass $$1 \ kg$$ produces an acceleration of $$1 \ m/s^2$$. 


Explain contact forces and field forces with example.

To set a stationary body into motion or to bring a moving body to a halt, Force on the body is necessary. Such forces are called contact forces that come with physical contact.
When an object experiences force without contact; such a force is called a field force.
Example: When a magnet is kept near some iron-pins, the iron-pins get attracted to the magnet and cling to it.


Water is filled as shown in the following containers. In which case the highest pressure is exerted on the base of the container? Why?
1844749_0dd3e53884944cd0bf5b421877f6c696.png

Water exerts the same pressure at the bottom of the container, regardless of its shape. Since height is constant, equal pressure is exerted in all cases.
$$P = \rho gh$$ (h = constant)


Answer the following question in brief.
How much pressure do we carry on our heads? why don't we feel it?

We carry atmospheric pressure of about $$10^5$$ Pa on our heads. We don't feel it because this atmospheric pressure is balanced by the pressure created by the air and blood inside our body. 


What is the force called by applying which the body changes its shape and size?

The force that changes the shape and size of the body is called the Deforming force.


Why does it happen?
The wall of a dam is broad at its base.

We know pressure of liquid increases with depth. So, the wall of a dam is made broader at its base so that it is able to withstand the heavy pressure exerted by the river water.


Why does it happen?
Fruits can easily be cut with a sharp knife.

Since the sharp edged knife makes lesser contact with the fruits to be cut, thus the pressure exerted by it on the fruit is very large. Because of this large pressure, fruits are easily cut with a sharp knife.


Write the word in the blank space.
The air pressure on our body is equal to __________ pressure

The air pressure on our body is equal to atmospheric pressure.


Define force.

Force is defined as that external agent acting on a body changes its state of rest or uniform motion along a straight line.


In the following example, try to identify the number of times the velocity of the ball changes:
A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team.
Also, identify the agent supplying the force in each case.

Explanation :
The velocity of the foot ball changes four times.
First when a football player kicks to another player, second when the player kicks that the football to the goal keeper. Third when the goal keeper stops the football. Fourth when the goal keeper kicks the football towards a player of his own team
Agents :
Agents supplying the force
$$1$$ . First case is First player
$$2$$ . Second case is Second player
$$3$$ . Third case is Goal keeper
$$4$$ . Fourth case is Goal keeper.














State the reason.
$$\text{It is easy to cut vegetables using a sharp knife.}$$

The sharp end of a knife has very less surface area. We know that  $$\text{Pressure (P)}=\dfrac{\text{Force(F)}}{\text{Area(A)}}$$. So, if the area is less then we can achieve a large pressure by applying some force. This large pressure will help us to cut the vegetables very easily.


Why cutting edge of knife is made sharper?

A sharp knife cut objects better because due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure and this large pressure cuts the object easily.


Did you apply any force on the stone to bring it down?

No we are not applying any type of force to bring it down. The gravitational force pulls the stone towards itself. So the stone moves downward.


Define force and mention the effect of force.

Force is a physical quantity that changes the state of motion of a body.
$$\text{Effects of force:}$$ 
  1. Change the state of motion of an object 
  2. Change the shape of an object.


Write the reason 
Constructing the building with a wide basement.

When the area increases the chance to depress the base in the soil is very less. So the building has more stability.


A sharp nail pierces more easily through wood than a blunt nail. What is the reason?

The surface area of the sharp tip of a nail is very small. Hence the pressure that it can exert on the wood is very great. That is why it pierces easily. For a blunt nail, the surface area of the tip is greater. Hence it can exert only a small amount of pressure.
$$\text{Pressure} = \dfrac{ \text{Force} }{ \text{Area} }$$


Classify the following situation into contact and non-contact forces.
A mango falling from a mango tree.

A mango falling from a mango tree.is an example of a non-contact force.
A force applied to an object by another body that is not in direct contact with it is known as a non-contact force. Non-contact forces come into play when objects do not have physical contact between them. 
A force that is applied by objects in contact with each other is known as contact force. The contact force acts on a point of direct contact between the two objects.


It is difficult to cut cloth using a pair of scissors with blunt blades. Explain.

Pressure is defined as the force per unit area. i.e.,
$$Pressure = \cfrac{Force}{Area}$$
Blunt blades have larger area compared to the sharp-edged blade.
According to above equation, if area is more, you have to apply more force to get the same pressure. Hence, it is difficult to cut the cloth using a pair of scissors with blunt blades.



The use of skis helps a person to slide smoothly over snow because the skis ____________.

The use of skis helps a person to slide smoothly over snow because the large area of the skis reduces the pressure on the snow and prevents the person from sinking into the snow.
The force acting on a unit area of a surface is called pressure. So, the larger the area, the smaller the pressure on a surface for the same force.


A girl weighing 50 kgf wears sandals of pencil heel of area of cross section 1 $$cm^2$$, stands on a floor. An elephant weighing 2000 kgf stands on foot each of area of cross section 25 $$cm^2$$, on the floor. Compare the pressure exerted by them.

Given,
Weight of girl $$F=50\ kgf=500\ N$$
Heel of area of cross-section $$= 1\ cm^2$$
Area of cross-section of both heels $$=1\times 2=2\ cm^2$$
Weighing of elephant $$=2000\ kgf=20000\ N$$
Area of cross-section of each foot $$=25\ cm^2$$
Area of cross-section all foots $$=25\times 4=100\ cm^2$$

Pressure exerted by girl    $$P_{girl} = \dfrac{F_{girl}}{Area} =  \dfrac{500}{2} = 250 \ N/cm^2$$......... (1)

Pressure exerted by elephant    $$P_{elephant} = \dfrac{F_{elephant}}{Area} =  \dfrac{20000}{100} = 200\ N/cm^2$$ ........(2)

On comparing above two-equation, Girl will apply more pressure on the ground. 


(i) Why do porter place a round piece of cloth on their heads when they have to carry heavy loads?
(ii) Why are shoulder bag provided with broad straps?
(iii) A bottle has four holes at its bottom and all of them are at the same depth. What would happen ?

(1) Porter place a round piece of cloth on their heads when they have to carry heavy loads so as to increase the area on this head. Hence, the pressure on the head gets reduced. As $$P=\dfrac{force}{area}$$

(2) Shoulder bag provided with broad straps. This is because if the straps are broader, then the area will increase and hence pressure gets reduced.

(3) Pressure at some height is given by $$P=\rho gh$$

Where $$\rho $$ is the density.

$$g$$is acceleration due to gravity.

$$h$$is height of the liquid column.

The hole at the bottom will exert less pressure as compared to others.



Describe a suitable experiment to demonstrate that a liquid exerts pressure sideways also?

Set up the apparatus as shown in the figure.
A deflated balloon is tied on the side tube of the container opened at one side. Now fill water in the container, balloon connected inflates due to the pressure of liquid exerted sideways. This shows that liquids exert pressure sideways also. 

1697277_1802539_ans_6589cb1f92f54c13ab6e8fd4c5dc2b10.png


Explain why, wooden (or concrete) sleepers are kept below the railway line.

Concrete or wooden sleepers are kept below the railway line so that the weight of passing train is spread over a large area of ground and the track may not sink into the ground.


Two women are of the same weight. One wears sandals with pointed heels while the others wear sandals with flat soles. Which one would feel more comfortable while walking on a sandy beach? Give reasons for your answer.

The woman with flat soles will feel more comfortable.
We know that $$Pressure=\dfrac{Force}{Area}$$
So, for the same force, the pressure will be more for less contact area. As the flat sole has more contact area as compared to the pointed sole, the pressure exerted by the flat sole will be less. So the flat show will not sink into the sand. that is why it is easier to walk on a sandy beach with flat shoes.


Describe an experiment to show that the liquid pressure at a point increases with the increase in height of the liquid column above that point.

1) Take a transparent glass tube or a plastic pipe and a rubber balloon.
2) Stretch the rubber sheet tightly over one end of the pipe.
3) Hold the pipe at the middle, keeping it in a vertical position as shown in the figure.
4) Pour some water into the pipe. The rubber balloon will bulge out. Note the height of the water column in the pipe.
5) Pour some more water. Observe again the bulge in the rubber balloon and the height of the water column. in the pipe.
6) Repeat the process and note the bulging in the rubber balloon and the height of the water column in the cylinder.

We will observe that the bulging of the rubber balloon increase with the increase in height of the liquid column. The pressure exerted by water at the bottom of the container depends on the height of its column.

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Deduce an expression for the pressure at a depth inside a liquid

Consider a vessel containing a liquid with density $$\rho$$.

Allow the liquid to be stationary

To calculate the pressure at a depth consider a horizontal circular surface PQ with area ;$$A$$, depth $$h$$ below the  free surface XY of the liquid
The pressure on  surface PQ will be due to the  weight of the liquid contained in cylinder PQRS of height h with PQ as its base and top face RS lying on the free surface XY of the liquids
Thrust exerted on the surface PQ

Volume of water column above the area PQ =$$ A * h$$
   Mass of the water column = $$m = A h {\rho}$$
   weight of the water column = $$m g = A h {\rho} g$$

$$pressure=\frac{Force}{Area}$$
$$pressure=\frac{Ah{\rho}g}{A}$$
$$pressure={h{\rho}g}$$

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Describe an experiment to show that liquid pressure depends on the density of liquid.

LIQUID PRESSURE DEPENDS ON THE DENSITY IF LIQUID:  take two balloons attached to one end of each of two identical glass tubes opened at both ends.
Fill tube (a) With milk and tube (b) With alcohol to the same height.
We notice that balloon attached to tube (a) bulged more than balloon attached to tube (b) This shows that the same height of milk exerts more pressure than alcohol.Since density of milk $$ ( 1.03) $$ is more than density alcohol $$ ( 0.8) $$ 

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Describe a simple experiment to demonstrate that a liquid enclosed in an vessel exerts pressure in all directions.

The experiment explained below demonstrate how a liquid enclosed in a vessel exerts pressure in all directions.
Procedure:
Fill a larger plastic bottle or a can with water
Set it on a horizontal surface 
Punch a few holes in the vessel at any point below the liquid surface Observation
Through each of the hole, water spurts out inference 
The spurting out of liquid indicate that the liquid exerts pressure at every point on the walls of the can/bottle
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Class 8 Physics Extra Questions