PHYSICS · Multi-part · A Level · Physics · Mechanics and materials · Mechanics and materials
Problem
Two identical solid blocks, Block A and Block B, each of mass 2.0kg, are connected by a light cord that passes over a small, frictionless pulley mounted on the ceiling of an elevator. Block A hangs inside the elevator cab; Block B hangs in the elevator shaft just outside the cab. The elevator is observed to travel upward at constant velocity relative to the Earth. Air resistance is negligible.
A physics student draws force diagrams for Block A, Block B, and for the combined system consisting of both blocks together. In each diagram the student includes gravity, tension, and a "force due to the elevator’s motion." The instructor questions the completeness and correctness of the diagrams.
Answer the following prompts in order.
(a) For Block A considered as an isolated object, identify all external forces acting on it. For each force, specify its direction.
(b) Repeat part (a) for Block B.
(c) Treating the two-block assembly (A + B) as a single system, list all external forces on the system and state the location on which each force acts.
(d) For the combined system, determine whether the net external force is zero. Provide a carefully reasoned argument that references the observed motion of the elevator.
(e) Critique the student’s inclusion of a "force due to the elevator’s motion" in each diagram. Explain why such a force should be accepted or rejected, and discuss the role of inertial reference frames in your argument.