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Momentum and impulse Edexcel International GCSE Physics revision

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In plain words

Momentum is "mass in motion": mass × velocity. A slow lorry and a fast cricket ball can both be hard to stop. In any collision the total momentum before equals the total after, as long as nothing outside pushes on them. And the longer a force has to change something's momentum, the smaller that force needs to be, which is how airbags and crumple zones work.

Three things to know

  1. Momentum = mass × velocity (p = mv). Its unit is kg m/s and it has a direction.
  2. In a collision with no outside forces, total momentum before equals total momentum after.
  3. Force = change in momentum ÷ time. Making a collision last longer makes the force smaller.

Worked example

A 2 kg trolley moving at 3 m/s hits a still 1 kg trolley and sticks to it. Find their speed afterwards.

  1. Momentum before = (2 × 3) + (1 × 0) = 6 kg m/s.
  2. Afterwards the mass is 3 kg, so 3 × v = 6.
  3. v = 2 m/s.

Tips and tricks

  • Momentum has a direction. Choose one direction as positive and give anything going the other way a minus sign.
  • For safety questions give all three steps: longer time to stop → smaller rate of change of momentum → smaller force.
6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Momentum and impulse: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. What is the momentum of a 50 kg runner moving at 8 m/s?
    • 6.25 kg m/s
    • 58 kg m/s
    • 400 kg m/s (the answer)
    • 3200 kg m/s

    p = mv = 50 × 8.

  2. Two trolleys collide and stick together. What is conserved, if no outside forces act?
    • speed
    • kinetic energy
    • total momentum (the answer)
    • velocity of each trolley

    Momentum is always conserved when there is no resultant outside force.

  3. Why do air bags reduce injury?
    • They reduce the change in momentum.
    • They increase the time taken to stop. (the answer)
    • They increase the force on the person.
    • They reduce the person's mass.

    A longer stopping time means a smaller force for the same change in momentum.

  4. A 3 kg object moving at 4 m/s hits and sticks to a 1 kg object at rest. What is their speed afterwards?
    • 1 m/s
    • 3 m/s (the answer)
    • 4 m/s
    • 12 m/s

    12 kg m/s shared by 4 kg gives 3 m/s.

  5. What is the unit of momentum?
    • N/kg
    • kg/m³
    • kg m/s (the answer)
    • m/s²

    Mass in kg times velocity in m/s.

  6. A force of 20 N acts on an object for 3 s. What is the change in momentum?
    • 6.7 kg m/s
    • 17 kg m/s
    • 23 kg m/s
    • 60 kg m/s (the answer)

    Change in momentum = force × time = 20 × 3.

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