Friction — Cornell Subtopic Notes — Notes

Cue	Notes
Three types of friction?	Static ( $f_s$ ), Kinetic ( $f_k$ ), Rolling ( $f_r$ ). Order: μ_s > μ_k >> μ_r.
When does static friction act?	Before relative motion begins. It adjusts to match the applied force up to μ_s N.
When does kinetic friction act?	Once the body is sliding. $f_k$ = μ_k N (constant, independent of speed).
Normal force on incline?	N = mg cos θ (NOT mg).
Net force on incline (kinetic)?	$F_{net}$ = mg sin θ − μ_k mg cos θ = mg(sin θ − μ_k cos θ).
Rolling friction μ_r?	Much smaller than μ_k. This is why wheels are used — rolling replaces sliding.
Angle of repose derivation?	At angle θ: mg sin θ = μ_s mg cos θ → tan θ = μ_s.
What is limiting friction?	The maximum value of static friction = μ_s N, just before motion starts.

Summary: Friction is a contact force opposing relative motion. Static friction self-adjusts (0 to μ_s N); kinetic friction is constant (μ_k N). On inclines, normal force is mg cos θ, not mg. Rolling friction is negligible compared to kinetic friction, making wheels essential for efficient transport.