# Inelastic Collision and kinetic energy?

## Homework Statement

A piece of taffy slams into and sticks to another identical piece of taffy that is at rest. The momentum of the two pieces stuck together after the collision is the same as it was before the collision, but this is not true of the kinetic energy, which is partly turned into heat. What percentage of the kinetic energy is turned into heat?

(my own addition) What if the initial piece of taffy was twice as massive as the one at rest when it collided?

## The Attempt at a Solution

Something with the equation for kinetic energy? Or is it conservation of momentum because of twice the mass and the same velocity? Not sure how the second relates to loss of energy for friction.

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gneill
Mentor
They're asking about kinetic energy being turned into heat. If some of it is turned into heat then that portion will no longer be contributing to the (bulk) motion of the system.

You have a perfectly inelastic collision. What's conserved? What's the KE before and the KE after collision?

Well, the KE isn't conserved since it's lost to heat... but momentum has to be conserved, but I don't see how the equation for that would lead to KE if I just figure out the mass and velocity changes.

If they were the same, but double the mass in the second case... velocity would have to change? And with mv I could find the KE?

Last edited:
gneill
Mentor
Well, the KE isn't conserved since it's lost to heat... but momentum has to be conserved, but I don't see how the equation for that would lead to KE if I just figure out the mass and velocity changes.
Look in your text, class notes, or on the web to investigate "inelastic collision". How do you determine the final velocity giving the initial masses and velocities of the colliding bodies?
If they were the same, but double the mass in the second case... velocity would have to change? And with mv I could find the KE?
Sure. You know the mass, so if you know mv you can find v, right? What's the expression for KE?