Two identical carts are at rest on a frictionless track and are held together by a…
PHYSICS · Single correct · AP · Physics 1: Algebra-based · Force and Translational Dynamics · Force and Translational Dynamics
Problem
Two identical carts are at rest on a frictionless track and are held together by a compressed spring. When the string holding the spring is cut, the spring pushes the carts apart. Which statement best describes the motion of the system’s center of mass immediately after the string is cut?
(a) It moves with a constant velocity in the direction of the cart that acquires greater speed, because the momentum of the carts carries the center of mass along.
(b) It oscillates back and forth about the midpoint, since the spring force acts alternately on the two carts as it expands.
(c) It remains at rest because all forces are internal to the system.
(d) It accelerates toward the cart that gains more speed, because that cart pulls the other one backward, producing an unbalanced force on the center of mass.