1. The problem statement, all variables and given/known data Two particles A and B having equal masses m are rotating around a fixed point O with constant angular speed ω .A is connected to point O with a string of length L/2 whereas B is connected to point A with string of length L/2 .Find the angular momentum of B with respect to A about O. O-------L/2-------A-------L/2--------B 2. Relevant equations 3. The attempt at a solution There can be two approaches 1.We find angular momentum of B w.r.t O ,say L1 = mω2L.Then we find angular momentum of A w.r.t O ,say L2 = (mω2L)/4.Since angular momentum is a vector , angular momentum of B w.r.t A should be vector difference of L1 and L2 =(3/4)(mω2L) 2.We find relative velocity of B w.r.t A =ωL/2 ,i.e we have considered particle A to be at rest .Then we find shortest distance between point A and line of motion of B which is L/2 . Now,angular momentum of A w.r.t B =(mω2L)/4 I feel approach 1 is the correct way of calculating angular momentum of a point with respect to a moving point ,but the correct answer is the one given by approach 2. Which is the right way ?