In a classic problem where a man walks along the length of a boat floating on the surface of water without friction, Should the net work done due to static friction between the man and the boat be zero. If the length of the boat is L, its mass being M, and the mass of the man being m, the displacements the boat and the man undergo could be calculated as mL / (M+m) and ML / (M+m) respectively. If F is the static friction between the boat and the man, it should be equal and opposite to the boat and the man. On calculating the net work done by the static friction on the boat - man system using F * displacement, even though the signs are opposite, it doesn't add up to zero. Is the work done by static friction on a system of particles always zero,if the static friction acts only as internal force between the particles? If so, why is it not apparently verified mathematically.