A 50 kg person at the left end of a 75kg boat is 6 m from shore. The person walks to the other end of the boat 3m away. How far is the person from the shore now? The system center of mass fixed in space so as the person moves right, the boat moves left. I know that the person moves 3 m to the right but how do I know how much the boat moves to the left? The answer is that the person will end up 4.2 m from shore, but how do I get there. If the person starts at 6 meter away and walks three meters towards the direction of the shore, the boat must travels a smaller distance relative to the person. This unit we're doing is on momentum and center of mass. I don't see how momentum will affect the result here, but maybe I am wrong. However, how does center of mass come into play here?