A fat person is standing on a light plank floating on a calm lake. The person walks from one end to the other on the plank. His friend sitting on the shore watches him and finds that the person hardly moves any distance because the plank moves backward about the same distance as the person moves on the plank. Explain.

Question

Philoid · Accepted Answer

Since the person is heavier than the plank, the center of mass of the person and the plank system effectively lies on the person. Since there is no external force on the system, the center of mass remains at the same position and so the observer on the ground feels that the person on the plank has not moved much.