Goal: Analyze the role of internal and external forces and the difference between static and kinetic friction.
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:
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Is consistent with Newton’s laws and can be done.
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Is consistent with Newton’s laws but cannot be done.
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Is not consistent with Newton’s laws and, therefore, cannot be done.
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Is not consistent with Newton’s laws but nevertheless can be done.
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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.