A horizontal force is applied to the left side of a pair of blocks that remain in…
PHYSICS · Single correct · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
A horizontal force F is applied to the left side of a pair of blocks that remain in contact as they accelerate across a frictionless table. A student models the two blocks as a single system of combined mass to find the acceleration a. Which statement best explains why this procedure yields the correct value for a?
(a) Because the applied force F is transmitted to both blocks, the system experiences a total external force 2F, but the combined mass is also doubled, so the acceleration remains F/(m1+m2). (This reasoning explains why treating them as one object seems to work.)
(b) Internal forces between the blocks form equal-and-opposite pairs that cancel when the blocks are treated as a single system.
(c) The normal force between the blocks is much smaller than the applied force, so ignoring it and treating the blocks as one object yields the same acceleration.
(d) The normal force between the blocks forms a third-law pair with the applied force F, so these forces cancel, allowing the two-block system to be treated as a single mass.