1. The problem statement, all variables and given/known data 2. Relevant equations The one in the middle with cot (big tetha) 3. The attempt at a solution I have no ideea how he finds the limiting cases for V<v0; v0<V and for V> v0/sqrt (1-v0^2) . The problem is from Landau course for theoretical physics, volume 2: The Classical Theory of Fields. page 35. I have been struggling with this problem for quite some time but made no real progress. Any bit of advice or even a full derivation step by step would be much appreciated. Is not a homework in the real sense btw. Thank you.