Normal derivative of derivated variable not working in Maple

Question

I have done this and I really have no idea what is wrong with Maple. Here is the equation and the Maple respond:

The text of error is: Error, invalid input: diff received diff(theta1(t), t), which is not valid for its 2nd argument.

I have no idea how to solve the equation…

Thanks in advance!

Answer 1

Try calling it as,

Physics:-diff(L, ...);

instead of as,

diff(L, ...);