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Kinetic energy and momentum

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Momentum - higher

You need to be able to calculate the force involved in changing the momentum of an object. Here is the equation you need:

Force is equal to: dividing the change in momentum by the time taken for the change

The force is measured in newtons, N. The time is measured in seconds, s.

Question

A 25 kg bicycle is travelling at 12 m/s. What force is needed to bring it to a halt in 5 s?

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Answer
  • Momentum at start = 25 × 12 = 300 kg m/s
  • Momentum at end = 0 × 12 = 0 kg m/s
  • Change in momentum = 300 - 0 kg m/s
The force needed is the change in momentum divided by the time taken for the change. So, 300 divided by 5 is 60 newtons

So a force of 60N is needed.

You should see that, for a given change in momentum, the longer the time taken, the smaller the force needed. This is the idea behind many car safety features. It also explains why it takes a long time to stop a super-tanker at sea, or to change its direction.

Watch this illustrated podcast on momentum to get a summary of how momentum and motion are related.

Now try a Test Bite - higher

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