When a sharp knock is given to a door, the moving finger has momentum. Once the door is struck, the momentum of the finger is reduced to zero in a very short interval of time, As a result the force imparted on the door is very large in a short interval of time, finger get hurted.

This large force acting for a short interval of time is called **impulsive force**. The product of force and time during which the force acts is called **impulse**.

**Impulse** = force × time

∴ **Impulse** = mass × acceleration × time = m a × t

(M(v-u)/t)×t = mv-mu

Thus, impulse can be defined as change in momentum. Like momentum, **impulse** is a **vector quantity**.

## Unit of Impulse

Impulse = force × time

∴ unit of mpulse in **S.I. system** is **N s** or **kg m s ^{-1} **and in C.G.S. system, it is

**dyne second**or

**g cm s**.

^{-1}## Dimensional Formula of impulse

Impulse = F × t = M^{1} L^{1} T^{1} × T^{1} = M^{1} L^{1} T^{1}

**Examples:** A cricket fielder lowers his hands while catching a ball. If the ball is catight. wathout lowering the hands, the fielder will hurt his hands due to a large force. When the ball is caught by moving the hand in the direction of motion of the ball, the duration of the umpact increases. As a result, the rate of change of momentum decreases, and thus, the force exerted by the ball on the hand is reduced.

An athlete taking a long jump or a high jump bends his knees before landing. By doing so, he increases the time of fall. This decreases the rate of change of momentum and this greatly reduces the impact of fall.

A blacksmith holds the rod in an anvil while striking it with a hammer thereby decreasing the time of contact, and increasing the **impulsive force**.

Thus, from the above facts, it is understood that the rate of change of momentum can be increased or decreased, respectively, by decreasing or increasing the time of contact.