Systems and Center of Mass Practice Test

A magnet attracts a steel cart on a low-friction track. The magnet is mounted to a second cart; the carts pull toward each other and eventually collide. Define the system as both carts (including the magnet). The magnetic forces between the carts are internal. Assume external horizontal forces are negligible. Initially both carts are at rest. What is the correct description of the center-of-mass motion?

A rocket in deep space ejects exhaust gases backward. Define the system as rocket + exhaust gases that have been expelled. Forces between the rocket and the exhaust are internal to this system, and external forces are negligible. Initially the system is at rest. As fuel burns and exhaust is expelled, which statement about the center of mass is correct?