I'm most of the way through this problem. I've already found the lagrangian equations of motion as a second order differential equation. I'm just stuck at the end... 1. The problem statement, all variables and given/known data Derive a first order differential equation for the motion of a hoop in a bigger hoop. 2. Relevant equations 3. The attempt at a solution I found the equation of motion to be: 0=g Sin(θ) + 3/2*R*Doubledot[θ] I think a conserved quantity is angular momentum = 3/2*m*k2*Dot[θ] How do I write θ as a first order difeq?