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A2L Item 029

Meta-Information
Description: 
Moving a chair by internal motions only.
Goal: 
Analyze the role of internal and external forces and the difference between static and kinetic friction.
Level: 
High School
Keywords: 
Dynamics
Keywords: 
Forces
Keywords: 
Friction
Keywords: 
Mechanics
Source: 
UMPERG

A person sits in an office chair with small wheels that swivel. The person claims she can move the chair across the room without touching anything but the chair, simply by kicking her legs outward. This claim:

  1. Is consistent with Newton’s laws and can be done.

  2. Is consistent with Newton’s laws but cannot be done.

  3. Is not consistent with Newton’s laws and, therefore, cannot be done.

  4. Is not consistent with Newton’s laws but nevertheless can be done.

  5. It is not possible to determine the correctness of the claim.

Commentary

Answer

(1); the process is possible because sudden impulse due to internal forces can exceed the static friction limit. By rapidly extending legs, alternated by slow retraction, the chair can be moved. Students are often aware of this but find it difficult to explain in terms of forces and dynamics.