# Hamilton-Jacobi equation in spherical coordinates

1. Dec 19, 2013

### Staff: Mentor

I was looking at the Wikipedia entry on the Hamilton-Jacobi equation, and was confounded by the equation at the beginning of the section on spherical coordinates:

http://en.wikipedia.org/wiki/Hamilton–Jacobi_equation#Spherical_coordinates

Shouldn't the Hamiltonian simply be
$$H = \frac{1}{2m} \left[ p_r^2 + p_\theta^2 + p_\phi^2 \right] + U(r, \theta, \phi)$$
?

2. Dec 19, 2013

### voko

In spherical coordinates, kinetic energy is $$T = {m \over 2} \left( v_{r}^2 + v_{\theta}^2 + v_{\phi}^2 \right) = {m \over 2} \left( \dot r ^2 + (r \dot \theta)^2 + (r \sin \theta \ \dot \phi)^2 \right)$$ By definition, $$p_r = {\partial T \over \partial r} = m \dot r \\ p_{\theta} = {\partial T \over \partial \theta } = m r^2 \dot \theta \\ p_{\phi} = {\partial T \over \partial \phi} = m r^2 \sin^2 \theta \ \dot \phi$$

3. Dec 19, 2013

### Staff: Mentor

Thanks a lot! I now realize I missed a square in my derivation