GCSE Combined Science revision · Physics: forces, waves and magnetism

Higher: momentum

Higher only

The idea

Momentum p = mv, measured in kg m/s. Momentum is a vector. In a closed system, the total momentum before an event equals the total momentum after it (conservation of momentum), for example in collisions and explosions.

Choose a positive direction.
Total momentum before = total momentum after.
Solve for the unknown.

Watch out Momentum is a vector: momentum in opposite directions has opposite signs.

See it worked

Worked example

A 2 kg trolley at 3 m/s hits a stationary 1 kg trolley and they move off together. Find their velocity.

Before: 2 × 3 + 1 × 0 = 6 kg m/s.
After: (2 + 1) × v = 6, so v = 2 m/s.

Answer 2 m/s

Your turn

Question 1Calculator allowed2 marks

What is the momentum of a 0.15 kg ball moving at 20 m/s, in kg m/s?

Question 2Calculator allowed2 marks

A 60 kg skater at rest pushes a 40 kg skater, who moves off at 3 m/s. What is the 60 kg skater's speed in m/s?

Question 3Calculator allowed1 mark

In a closed system, total momentum:

In past papers

18 official questions in 18 papers since 2018. The latest:

All GCSE Combined Science past papers

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