Friction is a property that makes objects resist being moved across one another. If two objects with flat surfaces are placed one on top of the other, the top object can be lifted without any resistance except that of gravity. But if one object is pushed or pulled along the surface of the other, friction causes resistance.
Friction has many important uses. It makes the tires of an automobile grip the road. It enables a conveyor belt to turn on pulleys without slipping. You could not walk without friction to keep your shoes from sliding on the sidewalk. It is hard to move on ice because the smooth surface of the ice produces less friction than a sidewalk, leading shoes to slip.
Friction also has disadvantages. It produces heat that may cause objects to wear. To prevent wear, oil and other lubricants are used to fill spaces between moving machinery parts. The lubricant reduces friction and makes the parts move more easily and produce less heat.
Kinds of friction.
There are three chief kinds of friction. Sliding or kinetic friction is produced when two surfaces slide across each other, as when a book moves across a table. Rolling friction is the resistance produced when a body rolls over a surface. The friction between an automobile tire and a street is rolling friction. Fluid friction is the friction between a fluid and a solid.
Laws of friction.
The basic law of friction says that the force needed to overcome friction is proportional to the total normal, or perpendicular, force pressing one surface against the other. That is, when the weight of a box being pulled across a floor is doubled, the force necessary to pull it doubles. When the box weighs four times as much, four times as much force must be used to pull it. The ratio between the force needed to move an object and the force pressing the surfaces together is called the coefficient of friction (C.F.). This ratio can be written with the formula C.F. = F/P.
For example, suppose a force of 30 pounds (F) is needed to pull a block weighing 80 pounds (P) across a flat surface. The coefficient of friction (C.F.) equals 30 divided by 80, or 0.375. In the metric system, the force would be measured in units called newtons. Suppose a force of 45 newtons is needed to slide a block weighing 12.2 kilograms. The block presses down with a force of 120 newtons. This is because gravity at Earth’s surface pulls with a force of 9.8 newtons for every kilogram an object weighs, and 9.8 times 12.2 equals 120. The coefficient of friction equals 45 divided by 120, or 0.375.
The coefficient of friction varies among materials. The C.F. of wood sliding on wood is between 0.25 and 0.50. Metal sliding on metal has a C.F. between 0.15 and 0.20. The frictional force due to rolling friction is about 1/100 as much as that due to sliding friction. But various conditions, including hardness, smoothness, and diameter of the materials, affect rolling friction. Oil reduces friction. The C.F. for iron rolled on oiled wood, for example, is much less than 0.018. The kind of surface has almost no effect when it is covered with oil or other liquids. The friction then depends on the viscosity of the liquid and the relative speed between the moving surfaces.
Friction between two objects arises from electrical attraction among their molecules. The energy lost to friction is typically transferred to the microscopic motion of the molecules. The resulting increase in motion can be felt as heat.
