# Rewrite Lagrangian's equation of motion as a 1st order difeq

1. Oct 20, 2011

### noon0788

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?

2. Oct 20, 2011

### mathfeel

You don't state all your variables clearly and it is difficult for me to figure out what system you are solving for.

If you are correct and$\frac{3}{2} m k^2 \dot{\theta}$ is indeed a COM (I don't know what $k$ is since you did not state in your original question and I am going to assume that it does not vary with time), then: $\ddot{\theta} = 0$, which would reduce the EOM to just $0 = g \sin\theta$.

3. Oct 20, 2011

### noon0788

Sorry, here's some clarification:

Basically, it's a hoop that's allowed to move in a larger fixed hoop without slipping.

k is the radius between the center of the free hoop and the bigger hoop that it's in. It does not vary with time.

If the EOM is 0=gsinθ, isn't that just trivial?

4. Oct 21, 2011

### mathfeel

There is no such thing as the radius between two things? You mean the distance between the center of the two loops? And I suppose your $\theta$ is the angle of the line that connects the two centers makes with the vertical? How did you eliminate the angular variable for the small loop? How did you apply the equation of constrain (which I think is what this problem is all about)? Why don't you start from the beginning and show us what you have done.

That is why I think you are wrong.