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A2L Item 136

Meta-Information
Description: 
Identify changes on the normal force on an object in an elevator.
Goal: 
Reasoning with dynamics
Level: 
High School
Keywords: 
Acceleration
Keywords: 
Dynamics
Keywords: 
Forces
Keywords: 
Gravity
Keywords: 
Mechanics
Source: 
UMPERG-ctqpe43

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
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.