force,work, power and energy

Linear Momentum
It is a common observation that we require more force to stop a heavier body as compared to a lighter body. If you catch a cricket ball as well as a table-tennis ball, then you will observe that more force is required to catch the cricket ball as compared to that required to catch a table-tennis ball. This happens because the mass of the cricket ball is more than that of the table-tennis ball. Hence, we can conclude that the force required to stop the motion of a body is directly proportional to its mass.



Similarly, more force will be required to throw the cricket ball as compared to that required to throw the table-tennis ball.


Let us assume that you have two cricket balls of the same mass. You throw one ball with a lesser force and the other with a greater force. What do you expect to happen? The ball that is thrown with the greater force will move with a greater velocity as compared to that thrown with the lesser force.

Hence, we can conclude that the effect of force on a body can be described with the help of its mass and velocity. To describe this, Newton defined the term momentum. He defined it as the product of the mass and velocity of a body, i.e.,

Momentum = Mass × Velocity

Or, p = m × v

Where, p = momentum; m = mass; v = velocity

The kind of momentum we have just defined is known as linear momentum as it is about the linear motion of a body. You will learn in later classes about angular momentum which deals with the rotational motion of a body.


Let us try to find the momentum of a cricket ball weighing 150 g moving at a speed of
20 m/s.

To find the momentum of the ball, we multiply its mass and speed.

Mass of the ball = 150 g = 0.15 kg

Velocity of the ball = 20 m/s

\Momentum of the ball = 0.15 kg × 20 m/s

= 3 kg m/s

Thus, the unit of momentum is kg m/s.