Question

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.

Answer 1

Since the plank is light its mass is negligible in comparison to the fat person. Combined center of mass of the system containing the fat person and the plank can be considered near the person. Since there is no external force on this system the center of mass of the system will remain at the same place. Due to this the friend on the shore finds the fat person hardly move.