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# Physics help watch

1. A ball of mass 0.1kg travelling at 4ms-1 hits a cushion and normally rebounds at 4ms-1. Calculate the force on impact if the time of impact is 0.1s

The answer is 8N but how, and which formula do we use
2. Force=rate of change of momentum
Change in momentum= mass x (final velocity-initial velocity)
If we take the final velocity with which it rebounds to be positive, the initial velocity is negative since it is travelling in the opposite direction.
So, change in momentum= 0.1 x [4-(-4)] = 0.1 x 8
Divide the answer by the time of impact to get the rate of change of momentum which is equal to force: (0.1 x 8)/0.1 = 8N
3. (Original post by Revision 99)
A ball of mass 0.1kg travelling at 4ms-1 hits a cushion and normally rebounds at 4ms-1. Calculate the force on impact if the time of impact is 0.1s

The answer is 8N but how, and which formula do we use
Use: Force = change in momentum / time

Momentum is mass x velocity
4. (Original post by lidzi_)
Force=rate of change of momentum
Change in momentum= mass x (final velocity-initial velocity)
If we take the final velocity with which it rebounds to be positive, the initial velocity is negative since it is travelling in the opposite direction.
So, change in momentum= 0.1 x [4-(-4)] = 0.1 x 8
Divide the answer by the time of impact to get the rate of change of momentum which is equal to force: (0.1 x 8)/0.1 = 8N
Thank you so much for a very clear and comprehensive explanation! Understood 👍
5. (Original post by Revision 99)
Thank you so much for a very clear and comprehensive explanation! Understood 👍

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