A lamp of mass m hangs from a spring scale that is attached to the ceiling of an elevator.When the elevator is stopped at the fortieth floor,the scale reads mg.What does it read while the elevator descends toward the ground floor at a constant speed?
A)more than mg
B)less than mg
C)mg
D)zero
E)This cannot be answered without knowing how fast the elevator is descending.

Question

Quizplus · Accepted Answer

When the elevator is stopped, the reading on the scale is simply the weight of the lamp, which is mg. The scale measures the force exerted on it by the lamp due to gravity. When the elevator descends at a constant speed, the lamp is still stationary relative to the elevator, and so the reading on the scale will not change. The force exerted by the lamp due to gravity is still mg, so the scale will continue to read mg. The speed of the elevator is not relevant in this case, as long as it is descending at a constant speed.

A Lamp of Mass M Hangs from a Spring Scale