1. The problem statement, all variables and given/known data I need to find the partial derivative of the action S with respect to the generalized coordinate q(tf) and according to my textbook, it should equal the generalized momentum p(tf). How can I derive this? 2. Relevant equations S = integral of L dt, with boundary ti to tf. (ti and tf are initial and final times) qi = 0 (initial generalized coordinate) 3. The attempt at a solution I took the partial derivative of both sides with respect to the generalized coordinate q, so the right side will have partial L partial q (which is equal to (p dot)) so I'm left with integral of (p dot) dt, but this is simple p(tf) - p(ti). Since qi = 0, p(ti) = 0. Hence partial S partial q(tf) = p(tf). Is this a possible solution? I have also uploaded my solution. Thanks.