Why don't inelastic collisions conserve energy?

2 Answers
Jun 3, 2018

Because some of the original energy goes to doing work, of some kind, such that it is lost to the system.



  • The classic is a bug splatting against the windshield (windscreen) of a car. Work is done on that bug, changing its shape, so some kinetic energy is lost.
  • When 2 cars collide, energy goes toward changing the shape of both cars' bodywork.

In the first example, that is a fully inelastic collision because the 2 masses remain stuck together. In the second example, if the 2 cars bounce off separately, that was an inelastic collision, but not fully inelastic.

Momentum is conserved in both cases, but not energy.

I hope this helps,

Jun 3, 2018


Energy is always conserved

But there is no law that says that Kinetic Energy is always conserved.

Inelastic collisions are defined as those in which Kinetic Energy is not conserved