Friction is the force resisting relative motion between surfaces. Static friction prevents motion from starting; kinetic friction acts during motion. Friction force = coefficient of friction (μ) × normal force (N). Higher μ means more friction.
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Pro Tip
On an incline, an object starts sliding when tan(θ) > μₛ. For μₛ = 0.5, sliding begins at θ = arctan(0.5) ≈ 26.6°. This is called the angle of repose — used in soil engineering.
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Did You Know?
Car braking uses friction between brake pads and rotors. ABS (Anti-lock Braking System) rapidly alternates between static and kinetic friction — static friction is higher, so ABS keeps you at the max braking force without wheel lockup.
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