A Force That Opposes the Motion of an Object: Which Friction?

Answer

The intended answer is C. Kinetic friction.

Kinetic friction acts when two contacting surfaces slide relative to one another. It points along the contact surface and opposite the relative sliding motion. In the simple dry-friction model, its magnitude is

$$f_k=\mu_k N,$$

where $\mu_k$ is the coefficient of kinetic friction and $N$ is the normal-force magnitude.

The other choices describe different situations:

• Rolling friction is resistance associated with rolling.
• Fluid friction acts when an object moves through a liquid or gas.
• Static friction acts when the contacting surfaces are not sliding relative to one another and can prevent sliding from starting.

The word moving alone is not a universal test for kinetic friction. For example, a wheel rolling without slipping can have a contact point that is instantaneously at rest relative to the ground. Pearson's item places the question in its kinetic-friction section and identifies C as the accepted answer, so the intended contact is sliding.