I've got a problem for which I'm trying to calculate $\ddot r$.
The problem is right here for the sake of reference.
So far, I've got that: $$\ddot r=\frac{d}{dt}v_r=\frac{dv_r}{dr}\frac{dr}{dt}$$
And since: $$\dot r= \frac{dr}{dt}$$
I simplified down to: $$\ddot r=\dot r \frac{dv_r}{dr}$$
Now I'm stuck, because I want to assume $v_r$ is a constant, in which case the whole $\ddot r$ term goes to zero. When I do this I get the wrong answer, however.
How could I take the derivative of $v_r$ with respect to $r$ in this situation?
Thanks in advance for any and all help!