Polar Coordinates Angular velocity

[jesuslovesu]

Homework Statement

A cameraman standing at A is following the movement of a race car traveling at a speed of 30 m/s. Determine the angular rate (theta dot) at which the man must turn to keep the camera directed on the car at theta = 30 degrees.

http://img143.imageshack.us/img143/8043/ohohohfc9.th.png
b = 20 m
a = 20 m
theta = 30 degs

Homework Equations

vr = r dot
vtheta = r*theta dot

The Attempt at a Solution

If only I knew r dot, I could figure this problem out.
I found r with the law of cosines (r = 34.641 m).
I was thinking of using
30 = sqrt( (rdot)^2 + r^2(thetadot)^2)
but I don't know r dot... where can I find the rate of change in r?

 

[mgb_phys]

Change in r only depends on change in phi which is constant and very easy to work out in terms of b. You can write r in terms of a and b.
So you can easily (he says!) write theta dot in terms of only a and b