**Goal:** Reasoning with dynamics

**Source:** UMPERG-ctqpe43

Block

m_{1} sits on block m_{2} and both sit on the floor of

an elevator at rest. When the elevator starts to move down, the normal

force on the upper block will …

- increase.
- remain the same.
- decrease.
- Cannot be determined

## Commentary:

## Answer

(3) As it starts the elevator must accelerate downward and so

will the upper block. The only forces on the block are gravity and the

normal force. The normal force must diminish so gravity can provide the

downward acceleration.

Students answering #2 may have interpreted the question to mean ‘as the

elevator moves’ and think that the elevator moves with constant velocity.