Kinetic friction opposes relative slipping
- 01
The tempting model
Drawing friction opposite the object's ground-frame velocity instead of opposite slipping at the contact.
- 02
Why it feels right
Friction is often summarized as 'opposes motion,' which is too vague.
- 03
Where it breaks · how to catch it
A block on a moving belt has friction drawn opposite its ground velocity without checking belt-relative motion.
- 04
The correct model
Kinetic friction opposes relative motion of the surfaces at contact. On a belt moving faster than the block, friction on the block can point forward.
- 05
Repair · see it, then test it
worked example A package lands on a conveyor belt moving right. Initially the package is slower than the belt. Which way is friction on the package?
Tempting move
Left, because the package moves right.
Correct move
Right, because the belt surface slips rightward underneath it.
Test the repair NL-12 · transferMake a fresh prediction before referring to the worked example.