Friction (Encyclopedia of Science)
Friction is a force that resists motion when the surface of one object slides over the surface of another. Frictional forces are always parallel to the surfaces in contact, and they oppose any motion or attempted motion. No movement will occur unless a force equal to or greater than the frictional force is applied to the body or bodies that can move.
Friction is often regarded as a nuisance because it reduces the efficiency of machines. It is also, however, an essential force for such items as nails, screws, pliers, bolts, forceps, and matches. Without friction we could not walk, play a violin, or pick up a glass of water.
Gravity and friction are the two most common forces affecting our lives. While we know a good deal about gravitational forces, we know relatively little about friction. Frictional forces are believed to arise from the forces of attraction between the molecules in two surfaces that are pushed together by pressure.
The surface of a given material may feel smooth. But at the atomic level, it is filled with valleys and hills a hundred or more atoms or molecules high. When one object is placed on top of another object, the pressure it exerts squeezes the hills and valleys in the two surfaces together. Molecules on the two surfaces tend to stick to one another, producing friction.
The laws of friction...
(The entire section is 551 words.)
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