Identify the direction of friction from contact motion

1. Frame the question

A rotating body can slide at its contact while its center moves forward. The no-slip relation vCM=Rω is a constraint to test, not a universal identity. To reason about friction, compare the instantaneous contact motion relative to the surface, not just the direction of the center’s motion.

2. Build the mechanism

For a disk moving right and spinning clockwise, the bottom point’s rotational motion relative to the center is leftward. If the center moves faster than this opposing rim motion, the contact slides right and kinetic friction acts left. That friction reduces rightward translation and produces a clockwise torque, increasing clockwise spin. If the disk is overspinning so the bottom slides left, friction acts right: translation speeds up while clockwise spin slows.

The numerical snapshot optionally displays the simple velocity difference vcontact=vCM−Rω to make the qualitative diagnosis explicit. AP Physics 1 requires the qualitative changes, not a quantitative time evolution or transition-time calculation for slipping. The tool deliberately does not integrate frictional motion or model rolling resistance.

Kinetic friction opposes relative contact motion and dissipates mechanical energy even when it increases the translational kinetic-energy part. In the overspinning case, a larger rotational-energy decrease can accompany a smaller translational-energy increase. At an instant with no slip, do not assign a kinetic-friction direction; static friction, if any, depends on other forces and constraints.

3. Work an example

For radius 0.2 m, center speed 1 m/s and clockwise spin 10 rad/s, the bottom rotational speed relative to the center is 2 m/s left. Contact therefore slips left at 1 m/s and friction acts right. With m=2 kg and I=0.04 kg m², the instantaneous energies are 1 J translation and 2 J rotation, but those two totals do not by themselves specify a later state.

4. Predict and investigate

Predict first: Can friction increase a body’s center-of-mass speed while total mechanical energy decreases?

Hold clockwise spin at 10 rad/s and compare center speeds 1, 2 and 3 m/s. Draw the center and rim contributions at contact, identify overspin/no-slip/underspin and predict the qualitative changes to translation and spin. Do not infer a transition time from this instantaneous snapshot. Retain settings and raw evidence in the lesson notebook.

5. Transfer and explain

Try a new practice case

As a diagnostic only, a disk has R=0.2 m, center speed 3 m/s right and clockwise spin 10 rad/s. Find signed contact speed, right positive.

Your numerical result (m/s) Check number

Reveal worked check (practice, not mastery)

This extra practice is not saved and does not award mastery. Record your first answer, explanation and whether you revealed help in the lesson notebook. Use the lesson’s saved question separately.

6. Reflect and retain evidence

Why is “friction always opposes the center’s velocity” an unreliable rule for a spinning body?

Retain your first response, system and axis choices, separate scalar energy and signed angular-momentum ledgers, assumptions and a changed-case prediction. Public worked checks are practice, not independent mastery evidence. Synthetic captures are not a performed physical experiment.

Framework and source notes

Original teaching prose and numerical cases. College Board AP Physics 1 CED, Unit 6 topic mapping: 6.5. Checked 30 October 2026. Planar signed analysis without required three-dimensional angular-momentum direction. Slipping evolution is qualitative, in accordance with the framework boundary; no rolling-resistance model. Framework cross-reference, not AP authorization.