I keep getting “Friction” questions wrong in Laws of Motion. What's the trap?
assuming friction always equals μN — static friction is only as big as needed, up to its limit.
The usual trap: assuming friction always equals μN — static friction is only as big as needed, up to its limit. How to avoid it: friction is a response force; it never exceeds μₛN before slipping. Go back to the definition — static friction adjusts itself up to a maximum μₛN to prevent motion; kinetic friction μₖN opposes sliding.
Key point
Friction
Common mistake
assuming friction always equals μN — static friction is only as big as needed, up to its limit.
Memory tip
friction is a response force; it never exceeds μₛN before slipping.
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