Question

A man stands at one end of the open truck which can run on frictionless horizontal rails. Initially, the man and the truck are at rest. Man now walks to the other end and stops. Then which of the following is true?

• A. The truck moves opposite to direction of motion of the man even after the man ceases to walk.
• B. The centre of mass of the man and the truck remains at the same point throughout the man's walk.
• C. The kinetic energy of the man and the truck are exactly equal throughout the man's walk.
• D. The truck does not move at all during the man's walk.

Answer 1

The correct option is B The centre of mass of the man and the truck remains at the same point throughout the man's walk.

since there is no external force on the system the COM of man-truck system will not change.
hence the truck will move in opposite direction until the man stops.
it will also stop if man stops.
also momentum will be conserved but no kinetic energy as the man is doing work.