hi there i am studying lagrange and hamiltonian mechanics. i came about this question from a previous test session please if anyone can help. the question is about a mass rotating on a rectilinear string attached without friction from a point A on the z axis. the length of the string is h = OA (O being the origin of the 3D system) the mass is free to move on the string as a pearl on a necklace for example. the rotational vel. "w" is given constant which makes the angle ф on the xy plane = wt this means that ф is not a degree of freedom i do not undersand however why the angle α between the string and the z axis is not. in the solution r the position vector from A to the mass M is the only degree of freedom. i think the answer would be simple and that if i scratched my head a bit i would know but i am still trying to figure it out in the mean time i tried asking here so.. if anybody can help me out it would be great thanks.