AQA A Level Physics

Revision Notes

4.5.2 Conservation of Momentum

The Principle of Conservation of Momentum

  • The principle of conservation of linear momentum states:

The total momentum before a collision = the total momentum after a collision provided no external force acts

  • Linear momentum is the momentum of an object that only moves in one dimension
  • Momentum is a vector quantity
    • This means oppositely-directed vectors can cancel each other out resulting in a net momentum of zero
    • If after a collision an object starts to move in the opposite direction to which it was initially travelling, its velocity will now be negative
  • Momentum, just like energy, is always conserved

Conversation of Momentum, downloadable AS & A Level Physics revision notes

The conservation of momentum for two objects A and B colliding then moving apart

External and Internal Forces

  • External forces are forces that act on a structure from outside e.g. friction and weight
  • Internal forces are forces exchanged by the particles in the system e.g. tension in a string
  • Forces which are internal or external will depend on the system itself, as shown in the diagram below:

External and internal forces on a mass on a spring, downloadable AS & A Level Physics revision notes

Internal and external forces on a mass on a spring

  • Systems with no external forces may be described as ‘closed’ or ‘isolated
    • These are keywords that refer to a system that is not affected by external forces
  • For example, a swimmer diving from a boat:
    • The diver will move forwards, and, to conserve momentum, the boat will move backwards
    • This is because the momentum beforehand was zero and no external forces were present to affect the motion of the diver or the boat

Worked Example

Trolley A of mass 0.80 kg collides head-on with stationary trolley B whilst travelling at
3.0 m s–1. Trolley B has twice the mass of trolley A. On impact, the trolleys stick together.

Using the conversation of momentum, calculate the common velocity of both trolleys after the collision.

Worked example - 1D momentum quesions solution

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