Wikipedia says:

Inelastic collisions may not conserve kinetic energy, but they do obey conservation of momentum.

Why does it use "may"? Does this imply that kinetic energy is sometimes conserved in an inelastic collision? As far as I know I haven't ever heard of such a situation. Is it possible?

First I thought may be potential energy is lost and changed to kinetic energy causing the final K.E. to increase, but I am unable to account for conservation of K.E. (Final k.e.=initial k.e.).

Also what then should be the perfect definition for an inelastic collision?

  • $\begingroup$ I cant see how thats mathematically possible $\endgroup$ – Lelouch Sep 14 '16 at 5:51
  • $\begingroup$ @Lelouch but my textbook says that kinetic energy is not conserved in an inelastic collision usually but not always. And asks the reader why? So I researched on Internet and got confirmed when Wikipedia too says "may", but iam unable to imagine such a situation. $\endgroup$ – JM97 Sep 14 '16 at 6:02
  • $\begingroup$ The 1st line of the article you quote states : An inelastic collision, in contrast to an elastic collision, is a collision in which kinetic energy is not conserved... $\endgroup$ – sammy gerbil Sep 14 '16 at 14:03

Yes, kinetic energy is always lost in an inelastic collision. This is by definition. A collision where kinetic energy is conserved is called "elastic". "Inelastic" means "not elastic", so kinetic energy is not conserved, by definition.

My guess is that the author of that sentence in Wikipedia was using the word "may" to express contrast between two ideas, not to express contingency. The sentence is roughly equivalent to

Although inelastic collisions do not conserve kinetic energy, they do obey conservation of momentum.

An everyday example of this use of the word "may" would be

Carl may have said "thank you", but he didn't mean it.

The speaker doesn't mean that it is uncertain that Carl said "thank you" - Carl did say "thank you". Instead, the word "may" is being used to introduce contrast, in this case contrast between Carl's words and his intent. In the sentence you quoted, the contrast is between kinetic energy not being conserved and momentum being conserved.

Of course, I did not write the Wikipedia article and cannot say with certainty that this is the intended interpretation.


In a collision between a bullet and a block of wood, both the bullet and the wood are deformed and the energy for this deformation is stolen from the initial kinetic energies of the two objects. That's the prototypical inelastic collision ("maximally inelastic" if the bullet becomes lodged in the target).

But there's usually another collision involved here: between the triggering mechanism inside of the gun and the ignition primer which ignites the propellant in the cartridge behind the bullet. Here again we have an interaction which conserves momentum but which involves a transformation between mechanical energy and chemical energy. The firing of the bullet by striking the igniter is also not an elastic collision, since the kinetic energy after the collision is different from the kinetic energy before. (Sometimes these are called "superelastic collisions," since kinetic energy is gained rather than lost.)


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