Force

# Objective

This topic gives an overview of;

• Force - A Push or a Pull
• Forces due to an Interaction
• Exploring Forces
• Change in State of Motion
• Change in Shape of an Object
• Contact Forces
• Non-contact Forces

# Force

You have learnt how objects move. 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.  what makes an object to 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? How do fielders stop a ball hit by a batsman? A hockey player changes the direction of the moving ball with a flick of the stick. 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?

# 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 motion of an object. These terms be replaced with one or more terms. Let us find out.

The Below table gives some examples of familiar situations involving motion of objects. You can add more such situations or replace those given here. Try to identify action involved in each case as a push and/or a pull and record your observations.

S.No Description of the situation
1 Moving a book placed on a table
2 Opening or shutting a door
3 Drawing a bucket of water from a well
4 A football player taking a penalty kick
5 A cricket ball hit by a batsman
8 Opening a drawer

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. The international unit used to measure force is newton. A force can change the shape and size of an object.

# Forces due to an Interaction

Suppose a man is standing behind a stationary car. Will the car move due to his presence? Suppose the man now begins to push the car, 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.

The below pictures shows three situations that may be familiar to you. In the Figure (a), both the girls appear to push each other while the pair of girls in Figure (b) are trying to pull each other. Similarly, the cow and the man in Figure(c) appear to pull each other. The girls in the two situations shown here are applying force on each other.

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.

# Exploring Forces

Let us try to learn more about forces.In the game of tug-of war, two teams pull at a rope in opposite directions. Members of both the teams try to pull the rope in their direction. Sometimes the rope simply does not move. The team that pulls harder, that is, applies a larger force, finally wins the game.

Forces applied on an object in the same direction add to one another. 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.

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. 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.

Choose a heavy object like a table or a box, which you can move only by pushing hard. Try to push it all by yourself. Can you move it? Now ask one of your friends to help you in pushing it in the same direction. Is it easier to move it now? Can you explain why? Next push the same object, but ask your friend to push it from the opposite side. Does the object move? If it does, note the direction in which it moves. Can you guess which one of you is applying a larger force?

# Change in 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 teammates 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.

## State of Motion

The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed. An object may be at rest or in motion; both are its states of motion.

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.

# Change in Shape of an Object

What do you conclude from the observations noted in below table.  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.

### Studying the Effect of Force on Objects

 Description of situation How to apply force Diagram Action of force Change in state of motion Change in shape Yes/No Yes/No A lump of dough on a plate Pressing it down with your hands No Yes Spring fixed to the seat of a bicycle By sitting on the seat No Yes A rubber band suspended from a hook/nail fixed on a wall By hanging a weight or by pulling its free end No Yes A plastic or metal scale placed between two bricks By putting a weight at the centre of the scale Tab No Yes

# 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 causedby the action of muscles in our body. The force resulting due to the action of muscles is known as the muscular force.

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.

Since muscular force can be applied only when it is in contact with an object, it is also called a contact force. It is the muscular force that enables us to perform all activities involving movement or bending of our body.

## Friction

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.  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 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

# Non-contact Forces

## Magnetic Force

Take a pair of bar magnets. Place the longer side of one of the magnets over three round shaped pencils or wooden rollers. Now bring one end of the other magnet near the end of the magnet placed on the rollers. Make sure that the two magnets do not touch each other. Observe what happens. Next, bring the other end of the magnet near the same end of the magnet placed on the rollers. Note what happens to the magnet placed on the rollers every time another magnet is brought near it.

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?

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

Take a plastic straw and cut it into nearly two equal pieces. Suspend one of the pieces from the edge of a table with the help of a piece of thread. Now hold the other piece of straw in your hand and rub its free end with a sheet of paper. Bring the rubbed end of the straw near the suspended straw. Make sure that the two pieces do not touch each other. Next, rub the free end of the suspended piece of straw with a sheet of paper. Again, bring the piece of straw that was rubbed earlier with paper near the free end of the suspended straw.

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.

## Gravitational Force

You know that a coin or a pen falls to the ground when it slips off your hand. The leaves or fruits also fall to the ground when they get detached from the plant.  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.

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.

# Summary

• Force could be a push or a pull.
• A force arises due to the interaction between two objects.
• Force has magnitude as well as direction.
• A change in the speed of an object or the direction of its motion or both implies a change in its state of motion.
• Force acting on an object may cause a change in its state of motion or a change in its shape.
• A force can act on an object with or without being in contact with it.

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