Impulse is simply a measure of the force applied for a specific time. Impulse = force x time and has units Ns (Newton seconds). Impulse is important in sport because many techniques, particularly throwing activities require the performer to apply as large a force possible for as long as possible. The area of a force time graph gives the impulse.

For example when hitting golf ball the player needs to strike the ball as hard as possible (applying a large force) as the ball will fly off in a very short time anyway. The golfer cannot really control how long the force is applied to the ball but they can increase the magnitude of size of the force.

However a Discus thrower needs to apply a force to the discus for as long as possible to put the most energy into the implement. The same is true for the Javelin thrower. The longer 'pull' they can apply to the object the faster it will leave the hand.

More examples:

• How can you improve an cricketers throwing technique to make them throw a ball further (increase the impulse)?

There are two ways they can increase impluse. One is applying a greater force so moving the arm faster which is likely to involve becoming stronger. The other way is to adjust technique so the force is applied over a longer time (or distance). So beginning the throw with the arm fully back and releasing as far forwards with a final flick of the fingers. However, be careful, there is little point increasing the the time the force is applied to the ball if the arm is moving slower (i.e. the forces are decreased).

• The cricketer applies a force of 20N to a ball for a 0.5 seconds. What is the impulse?

Impulse = force x time

I = 20 x 0.5 = 10Ns

•  A hammer thrower applies a force of 30N for 1 second doing 1 turn and 30N for 3 seconds doing 4 turns. What is the difference in impulse?

I (1 turn) = 30 x 1 = 30 Ns

I (4 turns) = 30 x 3 = 90 Ns

So we can see that three times more impluse is generated by doing three turns as opposed to four turns. The force is applied to the hammer for longer increasing impluse.

• Use your knowledge of impluse to explain whether a tennis ball struck with a backhand without following throw with the racket will hit the ball harder than a backhand where the players follows the shot through?

## Force time graphs

Force time graphs are often used to illustrate impulse. The area of the graph = impulse.

The graph below shows the force time plot for a vertical jump. When the athlete is standing still only weight from gravity is acting down. As they bend their legs ready to jump the force on the ground is negative and then when they explode upwards into the jump the force becomes positive again.