A wheel on an extremely low friction slanted surface will slide without rotating. However, if the same surface is rough and having some friction the wheel will roll down (with rotation).

Question

b. A wheel on an extremely low friction slanted surface will slide without rotating. However, if the

same surface is rough and having some friction the wheel will roll down (with rotation). Explain

why, using free body diagram.

Answer 1

Sketch a FBD for wheel on ramp. The weight vector points straight down from the center of the wheel, thus having a component parallel to the ramp. The normal force is perpendicular to the ramp, therefore equals the other component of weight, not the full weight. In the first case there are no other forces, and it slides down because of the parallel portion of the weight. In the second case, with friction always opposing motion, there is the friction force from the contact point, pointing up the ramp. This static friction (at any instant) causes rotation.