An elevator is being lifted upward at a constant speed by a steel cable. All frictional forces are neglected. In this situation, forces on the elevator are such that
A) the upward force by the cable is smaller than the downward force of gravity.
B) the upward force by the cable is equal to the downward force of gravity.
C) the upward force by the cable is greater than the downward force of gravity.
D) None of these. (The elevator goes up because the cable is being shortened, not because an upward force is exerted on the elevator by the cable.)

Question

Quizplus · Accepted Answer

According to Newton's third law of motion, the elevator and the earth exert equal and opposite forces on each other (action and reaction). The force of gravity pulls the elevator downwards, while the steel cable exerts an upward tension force on the elevator to balance the gravitational force. Since the elevator is moving at a constant speed, the tension force in the cable must be equal to the force of gravity acting on the elevator. Therefore, the upward force by the cable is equal to the downward force of gravity.

An Elevator Is Being Lifted Upward at a Constant Speed