1. The problem statement, all variables and given/known data This question is from Kleppner and Kolenkow's mechanics book, problem number 6.13: Mass m is attached to a post of radius R by a string. Initially it is distance r from the center of the post and is moving tangentially with speed v0. In case (a) the string passes through a hole in the center of the post at the top. The string is gradually shortened by drawing it through the hole. In case (b) the string wraps around the outside of the post. What quantities are conserved in each case? Find the final speed of the mass when it hits the post for each case. 2. Relevant equations 3. The attempt at a solution Okay, in part (a), I believe I have the answer, but I would like to make sure. Since the rope is being pulled through a hole at the center of the post, the tension force is central (radially inward). Therefore the torque about the center of the post must be zero; from there, we may conclude that angular momentum (mvr) is conserved. Thus, v0r = vfR and the problem is solved. Part (b), however, is different. Since the string is wrapping around the loop, the tension is not radial; thus, there is a torque on the mass about the center of the post, and angular momentum is not conserved. Clearly neither linear momentum nor energy are conserved. From here, I am not sure where to go. I could use the work-kinetic energy theorem, but this would involve a line integral about a path that I do not know... Any help on this problem would be greatly appreciated. Thanks.