# Class 8 Science NCERT Solutions for Chapter – 11 Force and Pressure

## Force and Pressure

Question 1.
Give two examples of each of the situations in which you push or pull to change the state of motion of objects.

(1) If a rubber ball or marble is placed on a very smooth surface and given a small push, it starts moving with some speed. If we keep an obstacle in the way of motion of the marble/ball, the marble/ball will stop moving at the obstacle. Here a push is given to the marble/ball at rest to change its state of rest to motion.

(2) If a child is placed on a hanging swing in the state of rest and the swing is pulled again and again by a rope, the swing will move to and fro with a speed. If the pull is bigger, the swing will move at a bigger distance. This is an example of a pull that changed the state of an object from the rest of the motion.

Question 2.
Give two examples of situations in which applied force causes a change in the shape of an object.

(1) If an inflated balloon is taken and pressed from two sides with a hand, its shape changed, i.e., the figure is disturbed. If the inflated balloon is pressed with more force, the balloon will burst.

(2) If a plate of steel is beaten with a hammer, after four-five big blows the plate’s shape will change.

Question 3.
Fill in the blanks in the following statements.

(a) To draw water from a well we have to _______ at the rope.
Pull

(b) A charged body _______ an uncharged body towards it.
Attracts (pulls)

(c) To move a loaded trolley we have to _______ it.
Pull

(d) The north pole of a magnet _______ the north pole of another magnet.
Repulses (pushes)

Question 4.
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 _______.
Shape

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

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

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

Question 5.
In the following situations identify the agent exerting a force and the object on which it acts. State the effect of the force in each case.

Hint :

(a) Squeezing a piece of lemon between the fingers to extract its juice.

Agent exerting – Object on which
Effect of force: Lemon juice is extracted out.

(b)Taking out paste from a toothpaste tube.

Fingers – Toothpaste tube
Effect: Toothpaste comes out of the tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall

Effect: The spring expands. Athlete’s legs, heart, and lungs.

(d) An athlete making a high jump to clear the bar at a certain height.

An athlete – Athlete’s legs, heart, and lungs.
Effect: He clears the bare at a certain height.

Question 6.
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 shape of the hot piece of iron changes. It flattens and becomes thinner than earlier. This is the effect of force due to hammering the piece of iron.

Question 7.
An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

This is an electrostatic force.

Question 8.
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.

The forces that act on the bucket are as follows :

1. The pressure of water contained in it exerted on its wall and on the base of the bucket.
2. Force of gravity exerted by earth.
3. Muscular force by my hand.

The water is contained in a bucket and the pressure is exerted by water on the walls of the bucket, which is sufficient to take it as a force. This force is counterbalanced by the force exerted by the walls of the bucket on water. As the bucket (with water) is held in my hand, so I overcome the force of gravity of the earth.

Thus, it can be concluded that there is no net effect of forces acting on the bucket full of water. That is why these forces do not bring a change in its state of motion.

Question 9.
A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

The two forces that are acting on the rocket immediately after leaving the launching pad are :

1. Force of gravity of earth acting down-ward
2. The frictional force produced by air particles.

Question 10.
When we press the bulb of a dropper with its nozzle kept in water, the air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) Pressure of water
(b) The gravity of the earth
(c) The shape of a rubber bulb
(d) Atmospheric pressure

(d) Atmospheric pressure

Question 1.
Do you recall how we can decide whether an object is moving faster than the other?

Yes, from the speed we can decide whether an object is moving faster than the other.

Question 2.
What does the distance move by an object in unit time indicate?

The distance moved by an object in unit time is known as speed.

Question 3.
Have you ever wondered what makes an object slow down or go faster, or change its direction of motion?

Force is the thing that makes an object to slow down or go faster or change its direction of motion.

Question 4.
What do you do to make a football move?

We kick a football to push ahead (by applying force).

Question 5.
What do you do to make a moving ball move faster?

The moving ball is pushed with a greater force to make it move faster.

Question 6.
How does a goalkeeper stop a ball?

The goalkeeper tries to apply force on the moving ball by diving or jumping up. The force applied by him can stop or deflect the ball, saving a goal being scored. If the goalkeeper succeeds in decreasing the speed of the ball to zero, the ball stops.

Question 7.
How do fielders stop the ball hit by a batsman?

Fielders try to pick/collect the ball hit by a batsman by applying force. By applying force, the fielder tries to make the speed of the moving ball zero. If he gets success in doing so, the ball stops.

Question 8.
What is a force?

A force is a push or pulls on an object.

Question 9.
What can a force do to bodies on which it is applied?

The body is pushed, pulled, thrown, flicked, or kicked when force is applied to it.

Activity 11.1

Question 1.
Table 11.1 gives some examples of familiar situations involving the motion of objects. You can add more such situations or replace those given here. Try to identify actions involved in each case as a push and/or a pull and record your observations in Table 11.1, One example has been given to help you.

11.1 Force – A Push or Pull

Question 10.
Do you notice that each of the actions can be grouped as a pull or push or both? Can we infer from this that to move an object it has to be pushed or pulled?

Yes, they can be grouped as a push or pull or both and from this, we can infer that to move an object, it has to be pushed or pulled.

Question 11.
When does a force come into play?

A force comes into play when we push, pull, kick, lift, flick, etc. any object.

Question 12.
I Learnt in Class VI that a magnet attracts a piece of iron towards it. Is attraction also a pull? What about repulsion between similar poles of two magnets? Is it a pull or push?

The attraction is also a pull because a magnet pulls other magnet or magnetic substance towards it. Repulsion is a push because in this case, a magnet pushes another magnet apart.

11.2 Forces are Due to an Interaction

Question 13.
Suppose a man is standing behind a stationary car [Fig. 11.2(a), Textbook Page 129]. Will the car move due to his presence?

No, unless there is an interaction between him and the car, it will not move.

Activity 11.2

Question 1.
Can you move it?

No.

Question 2.
Is it easier to move it now? Can you explain why?

Yes, it is easier to move now, because the two of us applied quite a bigger force.

Question 3.
Does the object move?

No, it does not move because both the forces are acting opposite to each other.

Question 14.
Fig. 11.5, is it not similar to the situation shown in Fig. 11.3 (6)?

Yes, it is similar to the situation shown in Fig. 11.3 (6).

Question 15.
Now recall what happened when you and your friend pushed the heavy box in the same direction in Activity11.2.

In this situation, we both added forces to one another and the box moved due to- greater force.

Question 16.
What did you observe in Activity 11.2, when both of you were pushing the heavy box from opposite directions?

When we pushed the object from opposite directions, we subtracted each other’s force which opposed each other’s effort and the body did not move.

11.4 A Force can Change the State of Motion

Activity 11.3

Question 1.
Does the ball begin to move?

Yes, it begins to move.

Question 2.
Push the ball again while it is still moving. Is there any change in its speed? Does it increase or decrease?

Yes, there is a change in its speed, it increases.

Question 3.
Does your palm apply a force on the ball? What happens to the speed of the ball now?

The palm applies a force that makes the ball to slow down.

Question 4.
What would happen if you let your palm hold the moving ball?

If the palm holds the moving ball, then the ball stops and comes to rest.

Question 17.
Paheli is curious to know whether the application of a force can only change the speed of an object.

Yes, apart from a change in the speed. an object, it (force) can change the direction of the moving body as well.

Activity 11.4

Question 1.
Does the ball continue to move in the same direction after it strikes the ruler?

No, the ball will come to rest or change its direction.

Question 2.
In each case, note your observations about the direction of motion of the ball after it strikes the ruler.

In this case, the ball will not stop, but it will change its direction with slower motion.

Question 18.
Can you give a few more examples of this kind?

In football, volleyball, and badminton, due to the application of a force, the change in speed, as well as direction, is noticeable.

Activity 11.5

Question 1.
Try to observe the effect of the force in as many situations as possible. You can also add similar situations using available material from your environment. Note your observations in Columns 4 and 5 of the Table.

Question 19.
What do you conclude from the observations noted in Table 11.2?

Change of shape took place in all the cases on the application of force.

11.6 Contact Forces

Question 20.
Can you push or lift a book lying on a table without touching it?

No, it is not possible as a contact, force is required in this case to lift a book.

Question 21.
Can you lift a bucket of water without holding it?

No.

Question 22.
Could it be a muscular force that does it?

Yes, it is the action of muscular force.

Question 23.
Where are these muscles located which make breathing possible?

The muscles are present in the lungs.

Question 24.
Can you list a few more examples of the force exerted by the muscles in our body?

In the following cases force is exerted by muscles :

1. The beating of the heart.
2. Movement of hands or legs.

Question 25.
Can you add some more such experiences?

1. A coin rolling on the floor stops.
2. A train toy stops after moving some distance when we stop applying the force.

Question 26.
What causes a change in their state of motion? Could some force be acting on them! Can you guess the direction in which the force must be acting in each case?

It is a force of friction that is acting on the bodies in motion. This force causes a change in the state of motion. The direction in which the frictional force acts is opposite to that of the motion of the body.

Question 27.
You may be wondering whether it is essential for the agent applying a force on an object to be always in contact with it.

No, sometimes an agent applies a force on a body without coming in contact with the body, e.g., magnet.

11.7 Non-contact Force

Activity 11.6

Question 1.
Observe what happens. Next, bring the other end of the magnet near the same end of the magnet placed on the rollers (Fig. 11.10, Textbook Page 135). Note what happens to the magnet placed on the rollers every time another magnet is brought near it.

In the first case, attraction took place. When another end of the second magnet is brought to the same end of the magnet on the roller, it moved away from it, showing that repulsion is taking place. In the second case, the first repulsion took place, and then attraction took place.

Question 28.
Does the magnet on the rollers begin to move when the other magnet is brought near it? Does it always move in the direction of the approaching magnet?

Yes, the attraction takes place only when opposite poles are brought nearby and repulsion takes place when similar poles are brought nearby.

Question 29.
What do these observations suggest? Does it mean some force must be acting between the two magnets?

The observations suggest the following :

1. The attraction takes place between opposite poles.
2. Repulsion takes place between similar poles.

Yes, some force is acting between the two magnets.

Question 30.
Do you have to bring the magnets in contact for observing the force between them?

No. A magnet can exert a force on another magnet without being in contact with it.

Activity 11.7

Question 1.
What do you observe?

The suspended straw attracts towards the straw in hand.

Question 2.
What do you observe now?

There is repulsion between the straws in this case, as both the straws have similar charges.

Question 31.
Leaves and fruits also fail to the ground when they get detached from the plant. Have you ever wondered why it is so?

The objects fall to the ground due to the earth’s gravitational force.

11.8 Pressure

Question 32.
Is there any relation between pressure and force?

Yes, the pressure is the force exerted on a unit area of a surface, i.e.,

Question 33.
Try to push a nail into a wooden plank by its head. Did you succeed?

No.

Question 34.
Try now to push the nail by the pointed end (Fig. 11.12, Textbook Page 137). Could you push it this time?

Yes, now the nail is pushed into the plank.

Question 35.
Try cutting vegetables with a blunt knife and then with a sharp knife. Which is easier?

It is easier to cut vegetables with a sharp knife than with a blunt one.

Question 36.
Do you get a feeling that the area over which the force is applied (for example, the pointed end of the nail) plays a role in making these tasks (pushing the nail into the plank) easier?

Yes, it is true that the area over which the force is applied is playing a prominent role in these tasks.

Question 37.
Can you explain now why shoulder bags are provided with broad straps and not thin straps? And, why the tools meant for cutting and piercing always have sharp edges?

Yes, due to broad straps, the area of contact of the load increases, and thus the pressure on the shoulder is reduced. Similarly, due to the sharp edges of cutting tools, the area of the contact decreases, thus pressure increases. So, it becomes easier to cut things.

Question 38.
Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts?

Yes, liquids and gases also exert pressure. The pressure exerted by liquids and gases does not depend upon the area of the base. It depends upon the height of the liquid or gas column of the container in which these are kept.

11.9 Pressure Exerted by Liquids and Gases

Activity 11.8

Question 1.
Does the rubber sheet bulge out?

Yes, the rubber sheet bulges out when the water column in the cylinder rises.

Question 43.
What happens when you open the mouth of an inflated balloon?

As soon as the mouth of the inflated balloon is opened, the gas starts rushing out of the balloon, which can be ascertained by putting a hand near the mouth of the balloon.

Question 44.
Suppose you have a balloon that has holes. Would you be able to? inflate it? If not why?

No, because the balloon cannot be kept inflated as air is escaping out through the holes,

Question 45.
Can we say that air- exerts pressure in all directions?

Yes, we can say that air exerts pressure in all directions.

Question 46.
Do you recall what happens to the air in the bicycle tube when it has a puncture? Do these observations suggest that air exerts pressure on the inner walls of an inflated balloon or a tube?

The air escapes either slowly or abruptly from the hole of the punctured cycle. Yes, air exerts pressure on the inner walls of an inflated balloon or a tube.

11.10 Atmospheric Pressure

Question 47.
How large or small is the atmospheric pressure?

It is very large.

Activity 11.11

Question 1.
Does it stick to the surface? Now try to pull it off the surface. Can you do it?

Yes, it sticks to the surface very hard. I can’t pull it.

Question 48.
Does it give you an idea of how large the atmospheric pressure is?