The Principle of Conservation of Momentum
- The principle of conservation of momentum states that:
The total momentum of a system remains constant provided no external force acts on it
- Therefore, for two objects in a collision:
the total momentum before the collision = the total momentum after the collision
- Remember that momentum is a vector quantity
- This allows vectors of opposing directions to cancel out
- Therefore, the momentum of the system as a whole is zero
- Momentum is always conserved over time
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
- Which forces are internal or external will depend on the system itself
Internal and External Forces for a Mass on a Spring
The spring force is internal to the system because the spring is part of the system, whereas weight (the gravitational pull of the Earth on the mass) is external to the system
- A system with no external forces acting can be described as a closed or isolated system
- For example, a swimmer diving from a boat:
- The diver will move forward, and the boat will move backward
- Therefore momentum is conserved
- Since the system is defined as the diver and the boat, any external forces acting on them that do not affect the momentum can be ignored