PHYSICS · Single correct · AP · Physics 1: Algebra-based · Force and Translational Dynamics · Force and Translational Dynamics
Problem
Two identical carts, A and B, are initially at rest on a frictionless track and are held together by a compressed spring. When the string holding them is burned, the spring pushes the carts apart. A student claims, “The forces the spring exerts on the carts are equal and opposite, so the carts should not move.” Which statement BEST corrects the student’s claim?
(a) The spring forces are unequal in magnitude until the spring reaches its relaxed length, so the temporary imbalance causes both carts to move apart.
(b) Although the forces are equal and opposite, each force acts on a different cart, so each cart experiences a non-zero net external force and accelerates.
(c) The spring forces are internal to the combined cart–spring system, so both carts accelerate because of an unbalanced external force supplied by the spring.
(d) Action–reaction forces cancel only when the interacting objects have different masses, so identical carts accelerate but would remain at rest if their masses differed.