I have a question about a classical physics problem. The original problem appeared as a homework problem in a physics book and it is really an extension of the problem that causes me an issue. The original problem went something like this: A frictionless puck travels, with linear velocity, v, in circular motion around an level air table, and the puck is held in circular motion by means of a string attached to its center. The string passes down through a frictionless hole in the center of the table and is held by a student so the length of the string above the table is R. While the puck is rotating, the student pulls in some string so the new length of the string above the table becomes R/2. What happens to the (a) linear velocity of the puck? (b) kinetic energy of the puck, and (c) where does the additional kinetic energy come from? The answers to these questions are simple: The linear velocity becomes 2v (because of conservation of angular momentum… M*V(1)*R(1) = M*V(2)*R(2) and thus the kinetic energy becomes 4 times as great. The additional energy is produced by the work done by the student as he pulls in the string. So conceptually, the additional energy comes from food eaten by the student and there are no difficulties with energy conservation. Now consider an extension that came to mind when I did this problem. Suppose we replace the hole in the table and the student with a thin post at the center of the table to which the string is firmly attached. We strike the puck perpendicularly to the string and it begins to rotate around the center pin. At the moment the string length is R, the linear velocity is v (just as before.) The string winds around the thin post so that eventually, the new radius is R/2. Again, the linear velocity needs to be 2v and again, the kinetic energy is four times as great as the original kinetic energy, but this time, I cannot attribute the increase in energy to a particular source. I’m sure the pole did not eat breakfast, and there is no engine or obvious source of energy. Can someone help me here?