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 between the molecules in two surfaces that are pushed together by pressure. The surface of a material may feel smooth, but at the atomic level it is filled with valleys and hills a hundred or more atoms high. Pressure squeezes the hills and valleys in the two surfaces together and the molecules adhere to one another. The actual contact area, from a microscopic perspective, is much less than the apparent area of contact as viewed macroscopically. As the weight of an object resting on a surface increases, it squeezes the two surfaces together and the actual area of contact increases. The actual contact area is believed to be proportional to the weight pushing the bodies together.
In addition to the adhesive forces between molecules, there are other factors that affect friction. They include the force needed to raise one surface over the high places of another; the fact that a rough region along a hard surface may "plough" a groove in a softer material; and electrical forces of attraction required to separate oppositely charged regions of the surfaces.
There are three laws that apply to friction. (1) The force of friction between an object and the surface on which it rests is proportional to the weight of the object. (The magnitude of the frictional force depends on the nature of the two surfaces.) (2) The force of friction between an object and the surface on which it rests is independent of the surface area of the object. (Remember, the actual contact area depends on the weight. If the weight remains constant so will the actual area of contact regardless of what the apparent area may be.) (3) The force of friction between an object and the surface on which it rests is independent of the speed at which the object moves as long as the speed is not zero.
The third law applies only to moving objects. Static friction, the force required to make an object at rest begin to move, is always greater than kinetic friction, which is the resistance to motion of an object moving across a surface. The reduction of friction that arises with motion is the result of fewer areas of contact once a body is in motion; the molecules are not in contact long enough to form firm bonds. Rolling friction for an object mounted on wheels or rollers is far less than kinetic friction. The reason that rolling friction is so small is probably the result of minimal contact area between wheel and surface, particularly if both are very hard, and because any molecular adhesions are pulled apart by vertical shearing rather than horizontal tearing.
In breaking molecular adhesions, molecular vibrations increase, causing a rise in temperature. You can easily verify this by simply rubbing your hands together. In machines, the adhesion and tearing of molecular bonds between surfaces causes wear. To reduce wear, we add a lubricant (often oil). The oil decreases the actual area of contact between surfaces. As a result, it reduces the wear associated with tearing surface molecules apart and thereby keeps heat and wear at lower levels than would otherwise be the case.