How to rearrange a second order differential equation? How would I go about rearranging a second order ODE?
For instance if I wanted to rearrange the below for $\theta$ how would I go about it?
$$\ddot{\theta}+\frac{g}{l}\sin \theta=0$$

I assume by $\ddot\theta=\dfrac{d^2\theta}{dt^2}$. 
I such cases, we introduce a new parameter like $p$ such that $p=\dfrac{d\theta}{dt}$, then
$$\ddot\theta=\frac{d^2\theta}{dt^2}=\frac{d}{dt}(\dfrac{d\theta}{dt})=\frac{dp}{dt}=\frac{dp}{d\theta}\frac{d\theta}{dt}=p\frac{dp}{d\theta}$$
thus your equation turns to
$$p\frac{dp}{d\theta}+\frac{g}{l}\sin\theta=0$$
This is a separable equation:
$$pdp=(-\frac{g}{l}\sin\theta) d\theta$$
which can be solved easily noting that $p=\dot\theta$. 
