On a horizontal surface, the kinetic friction force magnitude is f_k = μ_k N. If the surface is horizontal and N = m g, what is f_k?

On a horizontal surface, the kinetic friction force magnitude is f_k = μ_k N. If the surface is horizontal and N = m g, what is f_k?

• A. μ_k m g
• B. μ_k N
• C. μ_s m g
• D. μ_k g

Answer: (A)

Kinetic friction is always μ_k times the normal force. On a horizontal surface with no vertical acceleration, the normal force equals the weight, which is m g. So the friction magnitude becomes f_k = μ_k N = μ_k (m g) = μ_k m g. This is the correct expression. The static friction coefficient μ_s would only apply before motion starts, not during sliding. And a form like μ_k g would miss the mass factor, so it isn’t correct.

Kinetic friction is always μ_k times the normal force. On a horizontal surface with no vertical acceleration, the normal force equals the weight, which is m g. So the friction magnitude becomes f_k = μ_k N = μ_k (m g) = μ_k m g. This is the correct expression. The static friction coefficient μ_s would only apply before motion starts, not during sliding. And a form like μ_k g would miss the mass factor, so it isn’t correct.