FORCE AND LAWS OF MOTION, class IX, part-2

 

                       NEWTON’S LAWS OF MOTION

Newton has given three laws to describe the  motion of bodies. These laws are knows as Newton’s law of motion. The Newton’s laws of motion give a precise definition of force and establish a relationship between the force applied on a body and the state of motion acquired by it.

                          NEWTON'S FIRST LAW OF MOTION  

                 

When we see some of the bodies or objects around us are at rest, they are stationary, whereas others are in motion. Newton's first law describes the behaviour of such bodies which are in a state of rest or of uniform motion in a straight line.

According to Newton's first law of motion

A body at rest will remain at rest, and a body in motion will continue in motion in a straight line with a uniform speed, unless it is compelled by an external force to change its state of rest or of uniform motion. It should be noted that by saying an external force, we means a force from outside the body.

Let us take few examples to make the first law of motion more clear.

1. Suppose a box is lying on the ground it is in the position of rest. The box will not move by itself that is it cannot change its position of rest by itself. It can change its state of rest only when compelled by the external force by our hands that is when

we lift the box from the ground. Thus the positive of rest of the box has been

changed by the external force of our hands.

The tendency of a body to remain at rest or stationary or if moving then it will continue moving in a straight line is called inertia.

Newton’s first law recognises that everybody or object has some inertia. Inertia is that property of a body due to which it resists a change in its state of rest or of uniform motion. Greater the inertia of a body, greater will be the force required to bring a change in its state of rest or of uniform motion.

In fact mass is a measure of the inertia of a body. If a body has more mass, it has more inertia.That is heavier objects have more inertia than lighter objects.

 For example a sand bag has greater inertia than a football. If we kick a sand bag it will not move because of its high inertia but it we kick a football it will move a long ways. A sand bag has more inertia than a football.

Thus, the inertia of a body depends on its mass.

We conclude that to overcome the inertia and make a body move form rest, we must apply an external force.

2. If we take a glass jar and place a square card on its mouth. A coin is then placed above on this card in the middle. Let us flick the card hard with our fingers. On flicking the card moves away but the coin drops into the glass jar.

 We will now explain how it happens. Initially both the card and the coin are in the state of rest. Now when we hit the card moves away from the mouth of the glass jar. The force of our flick however, does not act on the coin, so the coin continues to be in its state of rest due to its inertia and when the card on which the coin had been placed moves away the coin falls into the glass jar because coin prefers the state of rest due to inertia

We will now consider the second part of the first law of motion which says that a body in uniform motion will continue to move unless a force compels it to change its state of uniform motion in a straight line.

Suppose a boy cycling a bicycle if he stops pedalling it. Than after some distance it will stop. At first sight it appears wrong. But if there were no air resistance and no friction to oppose the motion of a bicycle would go on moving forever. It would not stop by itself.

We will now describe some everyday observations which are based on the property of inertia of a body due to which it resists a change in its state of rest or of motion.

1. When a hanging carpet is beaten with a stick, the dust particles start coming out of it. This is because the force of stick makes the carpet move to and fro slightly but the particles tend to remain at rest or stationary due to their inertia and hence separate from the carpet.

2. When a tree having flexible stem is shaken vigorously, its fruits and leaves fall down. This is due to the fact that when the tree is shaken, it moves to and for slightly but its fruits and leaves tend to remain at rest or stationary due to their inertia and hence detach from the tree and fall down.

3. We have seen that when a car or bus starts suddenly the passengers fall backward. This is due to the fact that because of their inertia, the passengers tend to remain in their state of rest or stationary state even when the car or bus has started moving. When a running car or bus stops suddenly the passengers get jerked forward. This is because due to inertia the passengers tend to remain in their state of motion.  

Newton’s first law of motion gives us a definition of force. It says that a force is something which changes or tends to change the state of rest or of uniform motion of a body.