Which term is the ratio of the force of friction to the normal force?

• A. Inertia
• B. Coefficient of Friction
• C. Work
• D. Power

Answer: (B)

Friction between two surfaces is described by a simple ratio: the frictional force is proportional to the normal force pressing the surfaces together. This ratio is called the coefficient of friction, often written as μ. It is defined by μ = F_f / F_N and is dimensionless, meaning the proportionality doesn’t depend on the contact area or the exact forces beyond how hard the surfaces are pressed together. For static friction, the maximum friction you can get before slipping is μ_s F_N, and when sliding, the frictional force is F_f = μ_k F_N, where μ_k is the kinetic coefficient of friction. The idea that friction scales with the normal force and has a constant ratio for a given pair of materials is what makes the coefficient of friction the correct term. The other concepts don’t describe this relationship: inertia is about resistance to motion, work is energy transfer from a force over a distance, and power is the rate of doing work.