The principle of conservation of momentum
- The principle of conservation of linear momentum states that:
The total linear momentum before a collision is equal to the total linear momentum after a collision unless the system is acted on by a resultant external force
- Therefore:
momentum before = momentum after
- Momentum is a vector quantity, therefore:
- opposing vectors can cancel each other out, resulting in a net momentum of zero
- an object that collides with another object and rebounds, has a positive velocity before the collision and a negative velocity after
- Momentum is always conserved
- For example:
- Ball A moves with an initial velocity of
- Ball A collides with Ball B which is stationary
- After the collision, both balls travel in opposite directions
- Taking the direction of the initial motion of Ball A as the positive direction (to the right)
- The momentum before the collision is
-
- The momentum after the collision is
- The minus sign shows that Ball A travels in the opposite direction to the initial travel
- If an object is stationary, like Ball B before the collision, then it has a momentum of zero
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
- Which forces are internal or external will depend on the system itself
- A system with no external forces acting can be described as a closed or isolated system
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