4.1.18

# Momentum 2

Test yourself

## 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.