# What happens to the kinetic energy of a system in a non-elastic collision? [duplicate]

I know that the kinetic energy of a system is NOT conserved in a non-elastic collision. But energy is supposed to be conserved, so where does all that energy go? Is it transformed into other forms like heat and sound?

• Yup, you got it!
– user93237
Commented Oct 26, 2015 at 4:08
• Commented Oct 26, 2015 at 5:44
• There are lots of related questions. Have a look though the results from this search. I haven't found a question that addresses exactly what happens to the kinetic energy, though as you suggest it mostly ends up as heat. Commented Oct 26, 2015 at 6:50
• So then a truly elastic collision is silent. Commented Oct 26, 2015 at 6:55

Energy in a closed system (no external forces acting on the system) is always conserved — kinetic energy is not necessarily conserved.

In one of my labs, we rammed little hover cars into each other on an air track. To test an inelastic collision, we attached velcro strips to the fronts of the cars. When they collided, they stuck and moved together in the same direction. Let's think about what happened.

If it weren't for the velcro, the cars would have bounced off each other. The velcro keeps them together through friction between all the little velcro hairs. This friction opposes the desired motion of the cars (they want to bounce away from each other), and friction is a force, so it does negative work on the cars. This decreases their kinetic energy.

Of course, friction can't decrease total energy. The kinetic energy lost to friction is converted to heat in the velcro (which is actually just an increase in the kinetic energy of the velcro molecules). You already have experience with friction generating heat by rubbing your hands together, and you know it takes work to do this because you get tired if you do it a lot, really fast.

So you were right, the kinetic energy is simply converted to other forms of energy. It's fun to think about how it's possible for momentum to be conserved if kinetic energy is not (the two cars lost kinetic energy without losing mass, so how could their momentum be conserved?).

If a body is not elastic (sad ball) the KE will deform the body and this change is irreversible, the KE will be transformed into heat, sound etc. and will not be available anymore as mechanical energy. In this video you can see the enormous difference between a sad and a happy ball of same mass and momentum. If the concept of impulse is not clearly explained there this answer can be of great help

Why is mechanical energy converted as total energy is conserved in inelastic collision? Kinetic energy is transformed into an exactly equal quantity of other forms of energy in inelastic collisions, therefore the total energy of the system does not change: KE is not conserved whereas momentum is, but energy in general is conserved anyway

Image taken from this answer to Why is there conservation of kinetic energy in elastic collision and not in inelastic collision?.