AS Daily A Level Physics question
In a lab, two low-friction trolleys are coupled by a rigid bar. A student pulls the front trolley with a horizontal force so that both trolleys speed up together along a straight track. Considering only the horizontal interaction at the coupling, which statement must be true at every moment while they accelerate?
Answer
The correct answer is C.
Correct: C — The force on the rear trolley from the front one equals the force on the front trolley from the rear one in magnitude, acting in opposite directions, even while they speed up. A confuses the external pull with the interaction pair at the coupling; the third-law pair there must be equal and opposite. B assumes acceleration makes the forces unequal, but third-law pairs are equal at every instant and act on different bodies. C correctly states Newton’s third-law relationship and that it is independent of whether the motion is speeding up or steady. D wrongly links equality to constant speed, mixing up net force (second law) with action–reaction pairs (third law).