Conservation of Momentum
Physics: Cambridge International A Level, topic 3.3.1
Elastic and Inelastic Collisions
Elastic collisions conserve kinetic energy. Inelastic collisions do not conserve kinetic energy. Momentum is conserved in both.
Elastic collision
- In an elastic collision, momentum and kinetic energy are conserved.
- Conservation of momentum:
- Momentum before = momentum after
- Conservation of kinetic energy:
- Kinetic energy before = kinetic energy after
Inelastic collision
- In an inelastic collision, momentum is conserved but kinetic energy is not.
- Conservation of momentum:
- Momentum before = momentum after
- Kinetic energy is not conserved:
- Kinetic energy before ≠ kinetic energy after
- Some of the original energy is dissipated (lost) as heat or other types of energy.
Explosion
- An explosion is simply a collision in reverse.
- Explosions are inelastic processes as the kinetic energy of the system always increases.
- The kinetic energy of the fragments comes from the internal energy of the system.
Momentum and Safety
If an object which has a lot of momentum is made to stop suddenly, then a large force is required because of the change in momentum.
Safety features
- Cars have safety features such as seat belts, air bags and crumple zones that absorb kinetic energy in collisions.
- These features reduce injuries to the people in the car by absorbing energy when they change shape.
- They increase the time taken for the change in momentum to happen, reducing the forces involved.
Test yourself on Conservation of Momentum
What happens to momentum in an elastic collision?
Safety features of a car
How do safety features in a car reduce the force felt by a person in a crash?