A uniform chain of mass M and lengthL is hanging from the table. The chain is in limiting equilibrium when l length of chain overhangs. It is slightly disturbed from this position. Find the speed of the chain just after it completely comes off the table
Now consider the part that is originally on the table. A piece that's right at the edge falls a distance L/2, but a piece that's at the end doesn't fall at all. On average the part that start's on the table falls a distance L/4. Therefore the potential energy change for this part is (L/2)(L/4)
Set this change in potential energy to 1/2mv2, and remember that pL = m:
1/2mv2 = (pgL2)/4 + (pg L2)/8 = (3/8) pg L2 = (3/8) mgL
v = (3gL/4)