Elastic collision

Beginner

Both momentum and kinetic energy are conserved.

Key relationcollision

Both momentum and kinetic energy are conserved, with e = 1. Objects bounce apart without permanent deformation.

Worked example

Billiard balls and gas molecules are nearly elastic.

Common mistake

Most real macroscopic collisions lose some kinetic energy.

In more detail

An elastic collision conserves both momentum and kinetic energy. The objects bounce apart without losing energy to heat or deformation.

Steel balls in a Newton's cradle approximate elastic collisions.
Atomic collisions in gases are approximately elastic, which underlies kinetic theory.
Steel balls in a Newton's cradle bounce back with almost the same speed, showing an elastic collision in action.

Are real collisions ever elastic?

Only approximately. Some energy is always converted to heat or sound.

How do I know a collision is elastic?

Check whether the total kinetic energy before equals the total after.

What makes a collision elastic?

Kinetic energy is conserved, so no energy is lost to heat, sound or deformation during the impact.