Recent content by dfj

Thanks! I think I understand it now: The derivative dS/dt is defined along each real "world line". Each point of the world line has a unique t, and L is defined at each point. Understanding in this way, of course dS/dt=L. No calculation, even no thinking is needed, as far as the picture is...

Thanks for your reply! The thing is, on page 139 of this book, just below equation (43.3), the sentences are Apparently, what these words tells me is that the positions of both the starting and ending points are fixed, and the time of the starting point is also fixed, while the time of...

A question on "the action as a function of the co-ordinates" in Landau 1 Landau Mechanics, volume 1, third edition, page 138-139 (section 43) I can understand equations (43.1) - (43.3), which talk about treating the ending point of the path as a variable. Then, they continue to treat the...