Friction
The force that resists surfaces sliding over each other; slippery materials such as PTFE have very low friction.
Friction is the force that resists one surface sliding over another. Even surfaces that look smooth are rough at a microscopic scale, and where their high points touch they catch and cling. Friction is what lets shoes grip, brakes stop a car and a knot hold. It also wears out moving parts and turns useful energy into heat: rub your hands together and you can feel it.
Engineers describe how grippy a pair of surfaces is with the coefficient of friction: the force needed to slide one over the other divided by the force pressing them together. A value close to 1 means very grippy; a value close to 0, very slippery. Static friction, which stops something starting to move, is usually larger than sliding friction once it is moving, which is why heavy furniture is hardest to shift at the start.
One of the most slippery solids known is PTFE, sold as Teflon and used for non-stick pans. Physics textbooks give its coefficient of friction against itself as about 0.04.
A common misconception is that friction depends on how much surface is in contact. For most ordinary dry surfaces it depends on the materials and on how hard they are pressed together, but hardly at all on the area of contact; a brick slides about as easily on its side as on its end.
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