This was a conceptual question on my physics exam: True or false: You drop a non-rotating disk onto a rotating disk. When they hit they slide against each other with friction for a few seconds before they both start rotating at the same rate together. Because of losses due to that friction, angular momentum will NOT be conserved. (No other information was given) I put True, the "correct" answer was False. Am I wrong? Won't energy be lost as heat and sound through friction?