1. The problem statement, all variables and given/known data A uniform chain with a mass of M and a length of L is put on a horizantal table in a way that half of it is hanging from the air. At the moment t=0 the chain is released from rest. 1. What is the speed of the chain as its tip will leave the table? 2. Answer question 1 again only this time the chain has a kinetic coefficients of friction q with the table. 2. Relevant equations Work & energy theorm 3. The attempt at a solution Well I tried to take the center of mass point of the half chain that's hanged in the air. If I take the height of the table to be my refference height then in t=0 you get that this cener of mass point is located at -L/4 with respect to the height of the table. When all of the chain is in the air (just as its last part leaved the table) this point is now located at -3L/4 since L/2 of chain dropped from the table during this period of time. But in the solution they say that for the final situation I had to look at the new center of mass point, which is now at the middle of the chain which means it's at -L/2 with respect to the table. I don't understand it - aren't you suppose to determine the potential energy difference between two situations through a fixed refference height and a fixed point at the body I'm looking at? How does it help me that for t=0 I looked at one point of the chain and in the final stage I looked at another point of the chain?