Static friction is not always at its maximum
- 01
The tempting model
Using f = μs·N whenever there is friction.
- 02
Why it feels right
The expression μsN is often presented without emphasizing that it gives a maximum, not the friction in every static situation.
- 03
Where it breaks · how to catch it
You wrote f = μs·N for a body that isn't on the verge of slipping.
- 04
The correct model
Static friction is a range: it takes whatever value (≤ μs·N) is needed for equilibrium. Only at the threshold of sliding does it reach μs·N.
- 05
Repair · see it, then test it
worked example A crate (weight 200 N, μs = 0.4) is pushed with 50 N and stays put. Find friction.
Tempting move
f = μs·N = 80 N.
Correct move
f = 50 N — exactly balancing the push, which is below the 80 N maximum.
Test the repair NL-04 · transferinclude NMake a fresh prediction before referring to the worked example.
Then practise it on the ladder
R4Static friction thresholdadds: Static friction adjusts up to a maximum; it is not automatically μs N.