Class 9 Science

# Momentum

Momentum is the combined effect of mass and motion of an object.

The product of velocity and mass is called the momentum. Momentum is denoted by p.

Therefore, momentum of object = Mass × Velocity.

Or, p = m × v

Where, p = momentum, m = mass of the object and v = velocity of the object.

Let us take following examples to understand the momentum:

• What happens when a fielder is hit by a cricket ball? The fielder may get injured. This happens because of momentum of the ball.
• Why does a bullet kill a person, in spite of of its small size? The bullet has huge momentum because it comes at very high speed. The huge momentum is enough to either kill or maim its target.
• Why does a person get severly injured when hit by a vehicle? The vehicle has big mass and is moving at good speed. The combined effect of big mass and high speed produces enough momentum to severely injure a person.

## Momentum and Mass and Velocity:

You have read that momentum is the product of mass and velocity (p = m × v) of an object. This means momentum is directly proportional to mass and velocity. As a result, momentum increases with increase of either mass or velocity of an object.

This means if a lighter and a heavier object is moving with same velocity, the heavier object will have more momentum than the lighter one.

If a small object is moving with great velocity, it has tremendous momentum. And because of momentum, it can harm an object severely. For example, a small bullet having a little mass has a potential to kill a bird.

It is said that overspeeding is highly dangerous. Vehicles at high speed create huge momentum than those at low speed. This is the reason speed limit should be observed on the road.

### Momentum of an object at rest

Let an object with mass m is at rest.

Since, object is at rest, hence it's velocity, v = 0

Now, we know that,

Momentum = mass × velocity

Or, p = m × 0 = 0

So, the momentum of an object at rest is equal to zero.

## Unit of momentum:

The SI unit of mass = kg

The SI unit of velocity = meter per second i.e. m/s

We know that, momentum (p) = m × v

Therefore,

p = kg × m/s

Or, p = kg m/s

Therefore, SI unit of momentum = kg m/s

### More Examples of Momentum

A rubber ball and a cricket ball are moving at the same speed. Which of them is going to hurt more? It is obvious that a cricket ball will generate more momentum than a rubber ball because the a cricket ball has bigger mass.

You have to make a choice while driving on the road. You observe a road roller and a bicycle heading towards you. Which of these you will prefer to collide with head on? Why?

If all the planets in the Solar System are moving at the same speed, then which planet is going to have the greatest momentum? Why?