The coefficient of restitution describes the elasticity of a collision:

  • 1 = perfectly elastic, kinetic energy is conserved
  • 0 = perfectly inelastic, the objects move at the same speed post impact

However, COR values > 1 and < 0 are also physically meaningful:

  • COR > 1, a collision where the impact adds energy (e.g. an explosion)
  • COR < 0, a partial collision where the objects partially pass through each other. Say, like this:

apple and bullet

Are there colloquial terms that characterize these types of collisions? Perhaps explosion does work well enough for the first case, but I can't think of anything that adequately describes the apple case.

(If inelastic is defined as COR != 1 it's probably broad enough but likely not very illuminating in practice.)

  • $\begingroup$ Maybe, reactions? Elastic, inelastic, endothermic, exothermic reactions? $\endgroup$ Commented Oct 25, 2011 at 20:07
  • $\begingroup$ ""# COR > 1, a collision where the impact adds energy (e.g. an explosion) "" An impact adds energy? Where does he get that energy from? $\endgroup$
    – Georg
    Commented Oct 26, 2011 at 9:46
  • 1
    $\begingroup$ @Georg well perhaps it's not very precise to say the impact is adding energy; rather, the impact is the catalyst for another form of energy to be released that then influences the post-impact mechanics. For example, if that apple was really a lump of high explosive, and if the act of getting hit with a bullet was enough to detonate it, then the detonation would convert previously stored chemical energy into kinetic energy, which would likely accelerate the post-collision particles/bodies and thus cause the observed COR to exceed 1. $\endgroup$
    – Adam Wuerl
    Commented Oct 27, 2011 at 15:17
  • $\begingroup$ Ahh, I see, this sounds better. $\endgroup$
    – Georg
    Commented Oct 27, 2011 at 15:25

1 Answer 1


For COR < 0 you can say perforating collision (or piercing or even crossing).

For COR > 1 one could use exergonic collision, but maybe that causes more confusion. This is taken from chemistry where there are exergonic reactions.

My two cents.


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