2.5 Friction Force

Static vs. Kinetic Friction

Friction opposes relative motion between surfaces. Static friction prevents an object from starting to move, adjusting up to a maximum value f_s,max = μ_s·N. Kinetic friction acts once sliding occurs, with constant magnitude f_k = μ_k·N, where μ_k < μ_s. This is why more force is needed to start motion than to maintain it.

Normal Force Dependence

Friction depends only on the normal force (N), not on surface area or velocity. On an incline, N = mg·cosθ (without other vertical forces), so friction changes as the angle changes. Always find N from Newton's second law perpendicular to the surface before computing friction.

Friction in Dynamics Problems

Friction always acts opposite to the direction of relative motion or impending motion, parallel to the contact surface. When solving problems, draw a free-body diagram, resolve forces along and perpendicular to motion, and use ΣF = ma. Static friction is self-adjusting: it equals whatever value keeps the object at rest, up to its maximum.

Key terms

Static friction

Friction that prevents an object from starting to move; adjusts up to f_s,max = μ_s·N.

Kinetic friction

Constant friction force opposing sliding motion, given by f_k = μ_k·N.

μ_s vs. μ_k

Coefficient of static friction is usually greater than the coefficient of kinetic friction.

Normal force (N)

The perpendicular contact force between surfaces; friction is proportional to N.

Direction of friction

Always opposes the direction of relative motion or impending motion, parallel to the contact surface.

Try an AP-style question

A block slides down an incline at constant velocity. What is μₖ?

1. tan θ
2. cos θ
3. sin θ
4. 1