A block P rests on top of a block Q

PHYSICS · Single correct · AP · Physics 1: Algebra-based · Force and Translational Dynamics · Force and Translational Dynamics

Problem

A 2 kg2\,\text{kg} block P rests on top of a 5 kg5\,\text{kg} block Q, which in turn rests on a horizontal floor. A string attached to block P passes over a light, frictionless pulley and supports a hanging 4 kg4\,\text{kg} mass R. While the system is in motion, the floor exerts a 78 N78\,\text{N} normal force upward on block Q. Which of the following forces is the action–reaction partner of this 78 N78\,\text{N} normal force? (a) The downward gravitational force exerted by Earth on block Q (b) The upward normal force that block Q exerts on block P (c) The downward force of block Q on the floor, of magnitude 78 N78\,\text{N} (d) The upward tension force that the string exerts on block P

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