# First law of motion

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## Why does an object move?

What are the laws that make it move the way it does? Sir Isaac Newton (1642 - 1721) answered these questions.

## First law of motion

A body continues in a state of rest or uniform motion in a straight line unless, it is acted upon by external forces.

Push a ball along the floor. It moves but comes to rest after some time, even when there is no apparent external force acting on it. But then there is an external force. This force is friction. If we remove friction the ball will not stop, unless another force were to make it stop.

## What is force?

First law of motion helps us answer this question.

Force is the push or pull, which changes the state of rest, of a body, or that of uniform motion of the body in a straight line.

Uniform motion implies that the body is moving with constant speed along a fixed direction.

Force has both magnitude and direction so it is a vector quantity.

## What is inertia?

First law of motion helps us to answer this question also.

Inertia: A body, on its own, is unable to change its state of rest or of uniform motion in a straight line. This property is its inertia.

Take a tumbler. Place a cardboard piece on it. Now put a coin on this cardboard piece. Pull out the cardboard with jerk. What happens? If you do it correctly, the coin will drop into the tumbler. Why? The answer is inertia of rest. The force was applied on the cardboard and the coin got left behind because of its inertia of rest. Can you think of some other examples. Reason out what happens to passengers standing in a bus when it moves suddenly. Another example: what happens when we shake off the dust from a cloth or a blanket?

Imagine that the bus, in the above example comes to a sudden halt. The passengers are thrown forward, unless they hang onto something. This is inertia of motion. The same bus ( a useful analogy ! ) gets moving again and goes around a corner at a fast speed. The passengers are thrown outwards. This is inertia of direction. Can you think of more examples? If you can and want them checked send them to me by e-mail or if you want me to send more examples let me know.

It takes more effort to stop a heavy truck than to stop a car both moving with the same speed, why? This is due to linear momentum.

## Linear Momentum (p)

This is the total quantity of motion in a moving body and is given by the product of its mass (m) and its velocity (v).

### p = m v

Mass is scalar and velocity is a vector so linear momentum is a vector. The direction of linear momentum is the same as the direction of the velocity of the body.

In the example of truck and car given above in the page, if both have the same speed, the truck will have greter momentum since its mass is greater than that of the car.