I keep going around in circles with this. a little hint would be nice.A particle P, moving in a plane, has polar coordinates $\displaystyle (r , \theta )$ referred to the fixed pole O and the initial line in the plane

Given that the particle moves under a force directed towards towards O, show that at time t $\displaystyle r^2 \frac{d \theta}{dt} = h$.

Where h is a constant

Many Thanks

Bobak