A2L Item 136

• Description: Identify changes on the normal force on an object in an elevator.
• Goal: Reasoning with dynamics
• Source: UMPERG-ctqpe43
• Keywords: Acceleration, Dynamics, Forces, Gravity, Mechanics

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The question for students:

Block m₁ sits on block m₂ 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 …

1. increase.
2. remain the same.
3. decrease.
4. Cannot be determined

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Commentary for teachers:

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.

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