What's conserved?

What’s conserved?

In an inelastic collision:

• Momentum is conserved
• Kinetic energy is not conserved

What does this mean?

• The sum of the individual objects’ momentums before the collision is equal to the sum of their momentums after the collision (no momentum is ‘lost’)
• The sum of the individual objects’ kinetic energies before the collision is greater than the sum of their kinetic energies after the collision (some kinetic energy is ‘lost’ to the surroundings, for example as heat or sound)

What happens to the objects?

• The velocity of each object is reduced after the collision, but they won’t be swapped.
• The total momentum of the two objects will be the same before and after the collision, but the total kinetic energy will be less after the collision than it was before.

How does it differ from an elastic collision?

The key difference is that some energy is lost from the system in an inelastic collision, whereas no energy is lost from the system in an elastic collision.

Perfectly inelastic collisions

In a perfectly inelastic collision, the two objects stick together after the collision, and move together at the same velocity.

That’s only possible though if the larger object is moving faster than (and towards) the smaller object before the collision, so that the smaller object can catch up to the larger object and stick to it.