According to Wikipedia, the relativistic Doppler effect is simply the classical Doppler effect for a stationary source, $1-v/c$, times the relativistic time dilation, $1/\sqrt{1-v^2/c^2}$ (where v is relative velocity) which simplifies nicely to $$\sqrt{\frac{1-v/c}{1+v/c}}.$$ It then goes on to ...

A sphere of radius $R$ is supported by a rope attached to a wall as shown in the below figure. The rope makes an angle $\theta$ with respect to the wall. The point where the rope is attached to the ball is such that if the line of the rope is extended it crosses the horizontal line through the ce...