Friction is the 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.

While friction is often regarded as a nuisance because it reduces the efficiency of machines, it is, nevertheless, 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, and while we know a good deal about gravitational forces, we know relatively little about friction. Frictional forces are believed to arise from the adhesive forces...