Momentum 2
Physics: AQA A Level, topic 4.1.18
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 Momentum 2
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?