Intelligent assessment and revision tools for OCR A Physics

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?

• A The force on the rear trolley from the front one is less than the force on the front trolley from the rear one, because the front trolley is being pulled by the student.
• B The force on the rear trolley from the front one is greater than the force on the front trolley from the rear one, because the pair accelerates forward.
• 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.
• D The two forces at the coupling become equal only after a short time, once the speed becomes steady; during acceleration the front-on-rear force is larger.