When a wheel slips: reason about the contact point
You will be able to: Qualitatively predict how kinetic friction changes translation and rotation during slipping.
Which way does friction act when a wheel skids or spins too fast?
Slide a nonspinning wheel to the right on a rough floor. Its bottom surface slips right, so friction on the wheel points left. That force slows the center but also gives a clockwise torque, starting the wheel spinning.
A useful starting point: Down a ramp: share energy between translation and rotation →
Words and symbols before equations
- Slipping
- The wheel’s contact surface moves relative to the floor.
- Kinetic friction
- Friction opposing relative sliding at contact.
- Under-spinning
- For rightward motion, too little clockwise rotation to cancel contact motion.
- Over-spinning
- For rightward motion, clockwise rim motion at the bottom exceeds center motion.
What this picture assumes
Qualitative rightward-moving, clockwise-spinning wheel on a stationary rough floor. No other horizontal forces. Friction is zero in the steady pure-rolling snapshot. Arrows show directions only; no quantitative slipping dynamics is modeled.
Connect the picture to the physics
Choose rightward motion and clockwise rotation for the examples. An under-spinning wheel slips right at contact: friction points left, slowing the center and increasing clockwise spin. Both changes tend to reduce the relative slip.
An over-spinning wheel slips left at contact: friction points right, speeding the center and reducing clockwise spin. Friction therefore does not always oppose center-of-mass velocity; it opposes relative motion at the contacting surfaces.
During slipping, do not impose v_CM=Rω. Kinetic friction converts mechanical energy to thermal energy across the contacting system. Here we reason qualitatively; precise time evolution during slipping and rolling resistance are beyond this course’s stated scope. Once no slip is reached, the friction law and constraints change.
| Feature | Ideal no-slip rolling | Slipping on a stationary surface |
|---|---|---|
| Contact motion | Instantaneously at rest | Moves relative to the surface |
| Speed constraint | v_CM = Rω in magnitude | Cannot assume v_CM = Rω |
| Friction type if present | Static | Kinetic |
| Energy dissipation at contact | None in ideal rigid model | Mechanical energy becomes thermal energy |
A worked example, step by step
A wheel moves right and spins clockwise too rapidly for pure rolling. Explain contact slip, friction direction and changes in center speed and angular speed.
- Clockwise rotation carries the bottom surface left relative to the center.
- Here that leftward contribution exceeds the center’s rightward motion: contact slips left.
- Kinetic friction on the wheel points right, so center speed increases.
- A rightward force at the bottom gives counterclockwise torque, reducing clockwise spin. Mechanical energy is dissipated overall even though center speed increases.
Friction opposes relative slipping at contact, not necessarily the center-of-mass motion.
Can friction speed up the wheel’s center while total mechanical energy decreases?
Compare with an explanation
Yes. In the over-spinning case, friction transfers some rotational energy into translation while dissipating some as heat.
Predict. Change one thing. Explain.
Compare three contact states: under-spinning, pure rolling and over-spinning. Predict friction direction first. These are qualitative snapshots, not a time simulation or a quantitative slipping-motion model.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Contact slip: right. Friction on wheel: left. Center slows; clockwise spin increases. Mechanical energy becomes thermal energy during slipping.
Qualitative rightward-moving, clockwise-spinning wheel on a stationary rough floor. No other horizontal forces. Friction is zero in the steady pure-rolling snapshot. Arrows show directions only; no quantitative slipping dynamics is modeled.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant physical relationship or contact condition to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA wheel slides right without initially spinning on a rough level floor. (a) State initial contact-slip direction. (b) State kinetic-friction direction. (c) Predict the changes in translation and rotation. (d) Explain why v_CM=Rω cannot be imposed at the beginning.
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Compare with the answer and four-point rubric
- 1 point: Contact slips right.
- 1 point: Friction on the wheel points left.
- 1 point: The center slows; clockwise spin increases.
- 1 point: Initially v_CM is nonzero but ω=0, so the equality fails: the wheel is slipping.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What motion does kinetic friction oppose?
Relative sliding of the surfaces at contact.
RECALL 2Can slipping use the no-slip relation v=Rω?
No.
RECALL 3What happens to mechanical energy during slipping?
Some becomes thermal energy in the contacting system.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
When a wheel slips: reason about the contact point
- No slip: contact instantaneously stationary relative to floor.
- Under-spinning rightward wheel: friction left, center slows, clockwise spin increases.
- Over-spinning rightward wheel: friction right, center speeds up, clockwise spin decreases.
Remember: Friction opposes relative slipping at contact, not necessarily the center-of-mass motion.
Conditions: Qualitative rightward-moving, clockwise-spinning wheel on a stationary rough floor. No other horizontal forces. Friction is zero in the steady pure-rolling snapshot. Arrows show directions only; no quantitative slipping dynamics is modeled.
Refresh Kid · Unit 6 · Objectives 6.5.C · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 6.5, objectives 6.5.C. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. Fall-2026 corrections also checked. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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