# Energy conservation in collisions

1. Apr 21, 2012

### Woopydalan

I was curious, why is energy conserved in elastic collision but not in perfectly inelastic collision? It said this in my textbook without giving any reason why.

2. Apr 21, 2012

### Nabeshin

Umm, isn't this the DEFINITION of elastic and inelastic collisions?

If you want a physical intuition beyond that, we think of elastic collisions as those where the two balls colliding are perfectly rigid. So when they hit each other, they just immediately start moving in a different direction, their accelerations being instantaneous. In an inelastic collision, as the balls hit they deform each other before moving off in different directions. This deformation leads to the generation of sound, heat, and frictional forces between the two, all of which dissipate energy. Being perfectly rigid, the hard-sphere collision does not suffer from this.

3. Apr 21, 2012

### truesearch

'Energy' is always conserved, it is 'Kinetic energy' that is not conserved in inelastic collisions because of the generation of heat, sound and permanent deformation.

4. Apr 21, 2012

### Woopydalan

ok well elastic collisions generate sound too....I didn't realize this was the definition, the book didn't make it seem like those were the definitions, they just stated that one conserves kinetic energy and the other doesn't, while both conserve momentum, without explaining it, leaving me confused.

5. Apr 21, 2012

### truesearch

You are correct to say that momentum is conserved in all collisions. Kinetic energy is only conserved in perfectly elastic collisions. If any other kind of energy is generated then it is not perfectly elastic.
If sound is produced the collision is not perfectly elastic.
I think it is fair to say that perfectly elastic collisions only occur between atoms and molecules.

6. Apr 21, 2012

### Woopydalan

Oh ok, so it's that a ''perfectly elastic'' collision has no sound, heat, or anything else. I think this is a simplification made for intro level, because we are having perfectly elastic collisions applying to more than atoms. I was thinking elastic, didn't realize there was perfectly elastic. It said elastic and perfectly inelastic without mention of perfectly elastic, but I think they mean to say perfectly elastic.