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Motion and Forces

Collisions, and force as the rate of change of momentum

  • 3 things to know
  • Paper 5

Overview

Momentum in coupling trucks, snooker balls, skaters and a recoiling rifle, force as change in momentum over time, and how crumple zones and air bags lengthen that time.

What you need to know

Answer each one before you reveal it.

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Key term

Two railway trucks collide and couple together. Describe what happens to the total momentum.

Key term

A moving snooker ball strikes a stationary one and slows down. Describe the momentum change in this collision.

Key term

Explain why a rifle recoils backwards when a bullet is fired forwards.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Two railway trucks collide and couple together. Describe what happens to the total momentum.

    Model answer

    The total momentum is unchanged, and it is now carried by the joined trucks moving as one.

  2. 2

    A moving snooker ball strikes a stationary one and slows down. Describe the momentum change in this collision.

    Model answer

    The momentum lost by the moving ball is gained by the ball it strikes.

  3. 3

    Two ice skaters stand at rest and push each other apart. Describe their momenta afterwards.

    Model answer

    They move in opposite directions with equal-sized momenta, so the total is still zero.

  4. 4

    Explain why a rifle recoils backwards when a bullet is fired forwards.

    Model answer

    The total momentum stays zero, so the rifle gains backward momentum equal to the bullet's forward momentum.

  5. 5

    A 3 kg trolley moving at 4 m/s collides head-on with a 1 kg trolley moving at 4 m/s the other way, and they stick together. Calculate their velocity afterwards.

    Model answer

    The total momentum is 12 − 4 = 8 kg m/s shared by 4 kg, so they move at 2 m/s the way the heavier trolley was going.

  6. 6

    State in words the equation that links force, change in momentum and time.

    Model answer

    Force = change in momentum ÷ time taken.

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Key Concepts of Physics
Motion and Forces
Conservation of Energy
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