Which statement describes friction most accurately?

Which statement describes friction most accurately?

• A. A force proportional to the normal force and independent of the contact area.
• B. A force proportional to the contact area.
• C. A force equal to the weight of the object.
• D. A force that increases with speed.

Answer: (A)

Friction is governed by the normal load rather than the visible contact area. In the common model, the friction force equals the coefficient of friction times the normal force: F_f = μ N. This means the friction scales with N, which for a horizontal surface is the weight mg, and for an inclined surface is mg cosθ. The apparent contact area doesn’t set the friction because real contact happens at microscopic asperities, and those contact spots carry the load without requiring a larger apparent area. The other ideas don’t fit the standard description: friction isn’t proportional to the contact area, isn’t simply equal to the weight (except in special cases where μ happens to be 1), and (for kinetic friction) does not necessarily increase with speed—the friction force remains roughly constant with speed in the usual dry-friction model.

Friction is governed by the normal load rather than the visible contact area. In the common model, the friction force equals the coefficient of friction times the normal force: F_f = μ N. This means the friction scales with N, which for a horizontal surface is the weight mg, and for an inclined surface is mg cosθ. The apparent contact area doesn’t set the friction because real contact happens at microscopic asperities, and those contact spots carry the load without requiring a larger apparent area.

The other ideas don’t fit the standard description: friction isn’t proportional to the contact area, isn’t simply equal to the weight (except in special cases where μ happens to be 1), and (for kinetic friction) does not necessarily increase with speed—the friction force remains roughly constant with speed in the usual dry-friction model.