The maximum static friction force between two surfaces is proportional to the normal force by which relation?

The maximum static friction force between two surfaces is proportional to the normal force by which relation?

• A. The maximum static friction equals μs N
• B. The maximum static friction equals μk N
• C. Friction is independent of the normal force
• D. Static friction always equals kinetic friction

Answer: (A)

Friction resists tangential motion and its maximum value grows with how hard the surfaces press together. That pressing force is the normal force, N. For static contact, the largest friction you can develop before slipping occurs is f_s,max = μ_s N, where μ_s is the coefficient of static friction. The actual static friction can be smaller than this up to the limit, matching the applied tangential load to keep the surfaces from moving. If the tangential load exceeds μ_s N, slipping begins and kinetic friction μ_k N comes into play during sliding. The other statements don’t fit because friction does depend on N, kinetic friction involves μ_k, and static friction doesn’t always equal kinetic friction.

Friction resists tangential motion and its maximum value grows with how hard the surfaces press together. That pressing force is the normal force, N. For static contact, the largest friction you can develop before slipping occurs is f_s,max = μ_s N, where μ_s is the coefficient of static friction. The actual static friction can be smaller than this up to the limit, matching the applied tangential load to keep the surfaces from moving. If the tangential load exceeds μ_s N, slipping begins and kinetic friction μ_k N comes into play during sliding. The other statements don’t fit because friction does depend on N, kinetic friction involves μ_k, and static friction doesn’t always equal kinetic friction.