Friction
A resistive contact force that opposes relative sliding motion or tendency of motion between surfaces.
Governing formula
fs ≤ μs N, fk = μk N
SI unit
N
In depth
Friction arises from microscopic surface roughness and intermolecular bonds. Static friction opposes impending motion up to a maximum limit, after which kinetic friction acts during active sliding.
Examples in the real world
Tire tread friction enabling a car to brake safely, or brake pads converting kinetic energy to heat.