# Falling Chain help! [closed]

I was going through Example 9.2 in Thornton and Marion's Classical Dynamics, and I am stuck on the Potential Energy part of the Question. How do they get the term at the top of the page on the right? I understand how to reach U0, but getting from there to U is really confusing me. P = linear mass density "M/l", length "l" and mass "M", with "x" being distance traveled by the falling end of the chain.

• What do you think the expression for $U$ should be? What exactly is confusing you? – sammy gerbil Dec 6 '20 at 20:58