4.5.2 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

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:

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