I do not understand this one step in deriving the angular momentum theorem for a system of particles. The vector angular momentum about the point Q, not necessarily the origin is.. [PLAIN]http://img27.imageshack.us/img27/8196/pfquestion.gif [Broken] I do not understand why the difference in the velocities of the point Q and k equals the DERIVATIVE of the momentum. If you bring the mass term into the expression, then shouldn't it be equal to just p? Why is it dp/dt??