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The coefficient of static friction is a number that is determined based upon the static forces of two objects, typically influenced by the electromagnetic forces determined by the materials from which the objects are made. It is a value used in static systems in which two or more objects are at rest together, and represents an indication of the force that will be required to make one of the objects begin to move. The coefficient of static friction is typically symbolized using the Greek letter “mu” and is different depending on the materials the objects may be made from.
Also called the static friction coefficient or static coefficient of friction, the coefficient of static friction is a numerical value that can be determined for objects consisting of various materials. In general, it is a ratio determined by establishing the force needed to produce movement or sliding between two objects, and the normal force exerted between the two objects. The force needed to begin movement when divided by the normal force will produce a numerical value, which is the coefficient of static friction for those objects, or the materials from which they are made. In a system at rest, such as a wooden block lying on a wooden board, the normal force is equal to the force of gravity and keeps the block from entering the board.
The lower the coefficient of static friction is for a pair of objects and the materials from which they are made, the more slippery those surfaces are together and the easier it will be for motion to begin between the surfaces. For example, the block of wood on top of a wooden board has a coefficient of static friction of somewhere between 0.25 and 0.5, which is fairly low. Teflon, one of the slipperiest materials available, has a coefficient of static friction with most other materials of about 0.04.
Rubber on dry concrete has a static coefficient of friction of about 1.0 which allows the tires on a car to stick to a road and make the car come to a stop. As one end of the previously mentioned wooden board is raised slightly, creating a slope, the wooden block will initially remain at rest and the system remains static. This is because the force pulling the wooden block down the slope is not yet enough to overcome the static friction between the two wooden surfaces.
At a certain angle, however, the force of gravity is great enough to overcome the frictional force between the two objects, and the wooden block begins to slide down the board. The gravitational force needed to move the wooden block on the board must be greater than the coefficient of static friction, multiplied by the mass of the block multiplied by the normal force. When an object is not at an angle, the normal force is equal to the gravitational force or weight of the object, but as the angle increases the normal force is reduced and at the proper angle, is no longer great enough to keep the system static. At that point, motion begins and the frictional force becomes kinetic friction, rather than static friction.