Ok, so here's another example Morin did in his book: Ok, so the thing that confuses me is when he used the velocity addition equation: Note:4c/5 and 3c/5 are their speeds relative to the ground. Where v2 is the velocity of the S' frame, and v1 is the velocity of the object moving in the S' frame. For the B's point of view, he used v1 as the velocity of A, and v2 as the velocity of B...Now, the thing that confuses me is that the velocity of B in B's frame should be zero, so why did he used v2 as non zero in B's frame? He seemed to just plug in numbers and don't explain why he used those numbers, can anyone explain to me why he used those numbers in? I would understand a bit if it's in the ground frame, v2 wouldn't be zero, but then v1 should be the velocity of A with respect to B not with respect to the ground.