I teach grade 12 physics and am about to introduce collisions. I am explaining that in elastic collisions, kinetic energy is conserved and in inelastic collisions, kinetic energy is not conserved. The kinetic energy in inelastic collisions can be lost to heat, sound and electromagnetic radiation.
As I think about it though, do heat, sound and light not also have kinetic energy? Heat is movement of particles which is kinetic energy. Sound is movement of air particles which is also kinetic energy. Lastly, if I am not mistaken, light (EMR) has a relativistic kinetic energy.
If you were to track every molecule and photon's kinetic energy after a collision, would kinetic energy be conserved in all collisions?
The only thing that makes me think it wouldn't would be somehow if the energy was converted into some form of potential energy. Like a collision where one object causes another object to increase in height therefore increasing the gravitational potential energy of the object.
Edit: Note, I am not teaching my students anything deeper than the basic macroscopic conception of collisions. While I was prepping, it just got me thinking about this.
Best Answer
The definition of an elastic collision between two bodies is one for which the kinetic of energy of the two bodies remains the same after the collision (see https://en.wikipedia.org/wiki/Elastic_collision). A collision can only be perfectly elastic if there is no heat, sound, or light (or anything else) generated as a byproduct of the collision -- any energy devoted into these forms must be subtracted from the total incoming kinetic energy, i.e. the total kinetic energy of the two bodies is not conserved. While light and sound do have kinetic energy, they carry away some fraction of the initial system KE, so the collision is not elastic by definition.
As I see it, the answer to the question "Are all collisions really elastic?", given the definition of elastic, is really no. Not only that, but there is really no example of a perfectly elastic collision on the everyday scale of matter. Even cases where the objects do not touch, for example a fly-by scattering between two stars, generates heat from gravitational tides and would thus be inelastic.