I've noticed in several of my problems involving conservation of momentum, that the rotational kinetic energy of a system is often not conserved. Consider a merry-go-round spinning at a constant rate, until we drop a mass so that it lands on the rim of the merry-go-round. The rotational inertia increases, and due to conservation of angular momentum, the rotational velocity will decrease. But you can use whatever values you like, the rotational energy also decreases. I was trying to figure out where the difference goes. My first thought was that if the merry-go round were floating in space, moving the block onto it would impart a small amount of kinetic energy to the whole thing, and that might account for the difference. But you can change the initial speed of the block, and the difference in rotational energies doesn't change so long as we don't change the block's mass. I can't explain where the rotational energy goes in the this example. And I'm sure I could come up with more of these angular momentum scenarios where rotational energy isn't conserved. What's going on?