In Class VII, you have learnt how objects move. Do you recall how we can decide whether an object is moving faster than the other? What does the distance moved by an object in unit time indicate? You also know that a moving object like a ball rolling on the ground slows down. Sometimes it may change its direction of motion. It is also possible that the ball may slow down and also change its direction. Have you ever wondered what makes an object slow down or go faster, or change its direction of motion?

Let us recall some of our everyday experiences. What do you do to make a football move? What do you do to make a moving ball move faster? How does a goalkeeper stop a ball? A hockey player changes the direction of the moving ball with a flick of the stick. How do fielders stop a ball hit by a batsman? (Fig. 11.1). In all these situations the ball is either made to move faster or slower or its direction of motion is changed.

We often say that a force has been applied on a ball when it is kicked, pushed, thrown or flicked. What is a force? What can it do to bodies on which it is applied? We shall seek answers to such questions in this chapter.

11.1 Force – A Push or a Pull

Actions like picking, opening, shutting, kicking, hitting, lifting, flicking, pushing, pulling are often used to describe certain tasks. Each of these actions usually results in some kind of change in the state of motion of an object. Can these terms be replaced with one or more terms? Let us find out.

Table 11.1 : Identifying Actions as Push or Pull

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

In science, a push or a pull on an object is called a force. Thus, we can say that the motion imparted to objects was due to the action of a force. When does a force come into play? Let us find out.

11.2 Forces are due to an Interaction

Suppose a man is standing behind a stationary car [Fig.11.2(a)]. Will the car move due to his presence? Suppose the man now begins to push the car [Fig.11.2(b)], that is, he applies a force on it. The car may begin to move in the direction of the applied force. Note that the man has to push the car to make it move.

Fig. 11.3 shows three situations that may be familiar to you. Can you decide who is pulling and who is pushing in these cases? In Fig. 11.3 (a), both the girls appear to push each other while the pair of girls in Fig. 11.3 (b) are trying to pull each other. Similarly, the cow and the man in Fig. 11. 3(c) appear to pull each other. The girls in the two situations shown here are applying force on each other. Is it also true for the man and the cow?

From these examples, we can infer that at least two objects must interact for a force to come into play. Thus, an interaction of one object with another object results in a force between the two objects.

11.3 Exploring Forces

Let us try to learn more about forces.

Have you ever seen a game of tug-of war? In this game two teams pull at a rope in opposite directions (Fig. 11.5). Members of both the teams try to pull the rope in their direction. Sometimes the rope simply does not move. Is it not similar to the situation shown in Fig. 11.3 (b)? The team that pulls harder, that is, applies a larger force, finally wins the game.

What do these examples suggest about the nature of force?

Forces applied on an object in the same direction add to one another. Now recall what happened when you and your friend pushed the heavy box in the same direction in Activity 11.2.

If the two forces act in the opposite directions on an object, the net force acting on it is the difference between the two forces. What did you observe in Activity 11.2 when both of you were pushing the heavy box from opposite directions?

Does it mean that the net force on an object is zero if the two forces acting on it in opposite directions are equal?

Recall that in the tug-of-war when two teams pull equally hard, the rope does not move in any direction.

So, we learn that a force could be larger or smaller than the other or equal to each other. The strength of a force is usually expressed by its magnitude. We have also to specify the direction in which a force acts. Also, if the direction or the magnitude of the applied force changes, its effect also changes.

In general, more than one force may be acting on an object. However, the effect on the object is due to the net force acting on it.

11.4 A Force can Change the State of Motion

Let us now find out what happens when a force acts on an object.

You might recall similar situations. For example, while taking a penalty kick in football, the player applies a force on the ball. Before being hit, the ball was at rest and so its speed was zero. The applied force makes the ball move towards the goal. Suppose, the goalkeeper dives or jumps up to save the goal. By his action the goalkeeper tries to apply a force on the moving ball. The force applied by him can stop or deflect the ball, saving a goal being scored. If the goalkeeper succeeds in stopping the ball, its speed decreases to zero.

These observations suggest that a force applied on an object may change its speed. If the force applied on the object is in the direction of its motion, the speed of the object increases. If the force is applied in the direction opposite to the direction of motion, then it results in a decrease in the speed of the object.

Let us consider some more examples. In a game of volleyball, players often push the moving ball to their team mates to make a winning move. Sometimes the ball is returned to the other side of the court by pushing or smashing it. In cricket, a batsman plays his or her shot by applying a force on the ball with the bat. Is there any change in the direction of motion of the ball in these cases? In all these examples the speed and the direction of the moving ball change due to the application of a force. Can you give a few more examples of this kind?

A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion. Thus, a force may bring a change in the state of motion of an object.

Does it mean that the application of a force would always result in a change in the state of motion of the object? Let us find out.

It is common experience that many a time application of force does not result in a change in the state of motion. For example, a heavy box may not move at all even if you apply the maximum force that you can exert. Again, no effect of force is observed when you try to push a wall.

11.5 Force can Change the Shape of an Object

What do you conclude from the observations noted in Table 11.2? What happens when you apply a force on an inflated balloon by pressing it between your palms? What happens to the shape of a ball of dough when it is rolled to make a chapati? What happens when you press a rubber ball placed on a table? In all these examples you saw that the application of force on an object may change its shape.

Having performed all the above activities, you would have realised that a force

 may make an object move from rest.

 may change the speed of an object if it is moving.

 may change the direction of motion of an object.

 may bring about a change in the shape of an object.

 may cause some or all of these effects.

Table 11.2 : Studying the Effect of Force on Objects

While a force may cause one or more of these effects, it is important to remember that none of these actions can take place without the action of a force. Thus, an object cannot move by itself, it cannot change speed by itself, it cannot change direction by itself and its shape cannot change by itself.

11.6 Contact Forces

Muscular Force

Can you push or lift a book lying on a table without touching it? Can you lift a bucket of water without holding it? Generally, to apply a force on an object, your body has to be in contact with the object. The contact may also be with the help of a stick or a piece of rope. When we push an object like a school bag or lift a bucket of water, where does the force come from? This force is caused by the action of muscles in our body. The force resulting due to the action of muscles is known as the muscular force.

It is the muscular force that enables us to perform all activities involving movement or bending of our body. In Class VII you have learnt that in the process of digestion the food gets pushed through the alimentary canal. Could it be a muscular force that does it? You also know that lungs expand and contract while we inhale and exhale air during breathing. Where are these muscles located which make breathing possible? Can you list a few more examples of the force exerted by the muscles in our body?

Animals also make use of muscular force to carry out their physical activities and other tasks. Animals like bullocks, horses, donkeys and camels are used to perform various tasks for us. In performing these tasks they use muscular force (Fig. 11.9).

Since muscular force can be applied only when it is in contact with an object, it is also called a contact force. Are there other types of contact forces? Let us find out.

Friction

Recall some of your experiences. A ball rolling along the ground gradually slows down and finally comes to rest. When we stop pedalling a bicycle, it gradually slows down and finally comes to a stop. A car or a scooter also comes to rest
once its engine is switched off. Similarly, a boat comes to rest if we stop rowing
it. Can you add some more such experiences?

In all these situations no force appears to be acting on the objects, yet their speed gradually decreases and they come to rest after some time. 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?

The force responsible for changing the state of motion of objects in all these examples is the force of friction. It is the force of friction between the surface of the ball and the ground that brings the moving ball to rest. Similarly, friction between water and the boat brings it to a stop once you stop rowing.

The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion. Since the force of friction arises due to contact between surfaces, it is also an example of a contact force. You will learn more about this force in Chapter 12.

You may be wondering whether it is essential for the agent applying a force on an object to be always in contact with it. Let us find out.

11.7 Non-contact Forces

Magnetic Force

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

You have learnt in Class VI that like poles of two magnets repel each other and unlike poles attract each other. Attraction or repulsion between objects can also be seen as another form of pull or push. Do you have to bring the magnets in contact for observing the force between them? 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.

Similarly, the force exerted by a magnet on a piece of iron is also a non-contact force.

Electrostatic Force

A straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper. Such a straw is an example of a charged body.

The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. This force comes into play even when the bodies are not in contact. The electrostatic force, therefore, is another example of a non-contact force. You will learn more about electric charges in Chapter 15.

Gravitational Force

You know that a coin or a pen falls to the ground when it slips off your hand. Leaves and fruits also fall to the ground when they get detached from the plant. Have you ever wondered why it is so?

When the coin is held in your hand it is at rest. As soon as it is released, it begins to move downwards. It is clear that the state of motion of the coin undergoes a change. Can this happen without a force acting on it? Which is this force?

Objects or things fall towards the earth because it pulls them. This force is called the force of gravity, or just gravity. This is an attractive force. The force of gravity acts on all objects. The force of gravity acts on all of us all the time without our being aware of it. Water begins to flow towards the ground as soon as we open a tap. Water in rivers flows downward due to the force of gravity.

Gravity is not a property of the earth alone. In fact, every object in the universe, whether small or large, exerts a force on every other object. This force is known as the gravitational force.

11.8 Pressure

You have learnt in Class VII that strong winds during a storm or a cyclone can blow away even the roof-tops. You also learnt that winds and cyclones are caused by the differences in air pressure. Is there any relation between pressure and force? Let us find out.

Try to push a nail into a wooden plank by its head. Did you succeed? Try now to push the nail by the pointed end (Fig. 11.12). Could you do it this time? Try cutting vegetables with a blunt knife and then with a sharp knife. Which is easier?

Do you get the 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 easier?

The force acting on a unit area of a surface is called pressure.

pressure = force / area on which it acts

At this stage we consider only those forces which act perpendicular to the surface on which the pressure is to be computed.

At this stage we consider only those forces which act perpendicular to the surface on which the pressure is to be computed.

Note that the area is in the denominator in the above expression. So, the smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail is much smaller than that of its head. The same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank.

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

Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts? Let us find out.

11.9 Pressure Exerted by Liquids and Gases

Note that the rubber sheet has been fixed on the side of the container and not at the bottom. Does the bulging of the rubber sheet in this case indicate that water exerts pressure on the sides of the container as well? Let us investigate further.

Can you now say that liquids exert pressure on the walls of the container?

Do gases also exert pressure? Do they also exert pressure on the walls of their containers? Let us find out.

I have seen fountains of water coming out of the leaking joints or holes in pipes supplying water. Is it not due to the pressure exerted by water on the walls of the pipes?

When you inflate a balloon, why do you have to close its mouth? What happens when you open the mouth of an inflated balloon? Suppose you have a balloon which has holes. Would you be able to inflate it? If not, why? Can we say that air exerts pressure in all directions?

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? So, we find that gases, too, exert pressure on the walls of their container.

11.10 Atmospheric Pressure

You know that there is air all around us. This envelop of air is known as the atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure. We know that pressure is force per unit area. If we imagine a unit area and a very long cylinder standing on it filled with air, then the force of gravity on the air in this cylinder is the atmospheric pressure (Fig. 11.17).

But, how large or small is the atmospheric pressure? Let us get an idea about its magnitude.

Fig. 11.17 : Atmospheric pressure is the force of

gravity on air in a column of unit area

When you press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because the pressure of atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. This activity might give you an idea about the magnitude of atmospheric pressure. In fact, it would not be possible for any human being to pull the sucker off the surface if there were no air at all between the sucker and the surface. Does it give you an idea how large the atmospheric pressure is?