Friction is a very powerful force. It exists in the world, sometimes hindering people, at other times helping people. The word “friction” is thrown around a lot, but what exactly is it? When a child first thinks of friction, he/she may imagine a person rubbing their hands together rapidly. This is partially true, because friction is being created. But, friction can be examined all the way from a microscopic level to real world applications.

Basic IdeasEdit

Although some surfaces may appear to be smooth, every surface is rough at a microscopic level. When two surfaces come into contact with one another, the crests of the surfaces momentarily bond. When the surfaces move, these bonds must be broken. When the surfaces shift the bonds continue an attraction. In essence, this is friction.

Static and Kinetic FrictionEdit

What if one were to push a heavy dump truck across the highway? It wouldn’t budge. Even though all energy would be applied to it, there would be no motion. This is due to the static friction force. This force is opposite in direction of the applied force, and equal in size. The person could push harder and harder, but if the truck doesn’t move, the friction force is also getting larger.

Eventually, an army of people come to help move the truck; the static friction can only grow so large. Finally, the crate begins to move. This is called kinetic friction force. Kinetic Friction Force is the might exerted from one plane to the other when the two are in motion.

Types of FrictionEdit

There are three general types of friction. If two solid objects touch and a force is used to move one object against the other, sliding friction results. Rolling friction is when a spherical or circular object comes in contact with a solid object. If a force is applied, the first object will begin to roll. The last type of friction is when a solid object comes into contact with a liquid or gas. If a force is applied to the object, friction results. Examples of fluid direction include an airplane flying in the clouds or a submarine, deep under the water.

Practical ApplicationsEdit

A simplified equation can be used to express the mechanics of friction. However, it is a simplified idea, the area of the surfaces and the speed of their relative motion are not taken into account. Overall, the friction depends on the surfaces in contact. The magnitude of the frictional force is relative to that of the forces pushing against eachother. This force, which is always perpendicular to the surface is called the normal force. The variable of the normal force is F .The equation appears as follows: f F=UF

f       n

U is a constant known as the kinetic coefficient of friction. It can be found for a multiude of materials in the following table:

Materials Kinetic Static
Rubber on Concrete (dry) .68 .90
Rubber on concrete (wet) .58
Rubber on asphalt (dry) .67 .85
Rubber on asphalt (wet) .53
Rubber on ice .15
Waxed ski on snow .05 .14
Wood on wood .30 .42
Steel on steel .57 .74
Copper on steal .36 .53
Teflon on Teflon .04

The object in motion is directly proportional to the normal force.

Major PlayersEdit

Much of Isaac Newton’s ideas are incorporated into the laws of friction. For example, Newton’s third law states that “if object A exerts a force on object B, then object B exerts an equal and opposite force back on object A.” This idea incorporates the balancing act that occurs when discussing friction. The force of friction and the force of motion display a relationship so that they work off of oneanother to create movement as discussed earlier.

Practice ProblemsEdit

If a 50 N object slides across a horizontal surface and a 12 N frictional force is present, calculate the coefficient of Kinetic friction.


First, it is important to know which information is given, and which information is necessary to find. The problem gives the normal force (50N) and the force of friction (12N). The missing link is the coefficient of friction. With simple algebra, and following the equation, the force of friction must be divided by the normal force. The answer comes to be .24.

If there is dry rubber moving along concrete and the normal force is 25N, what is the frictional force?


In this problem, the normal force is given (25N) and the coefficient of friction is given, if one looks at the table, it is .68. The two numbers are then multiplied to get the Force of friction, which is 17N.


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Weisstein, Eric W. . "Issac Newton." 2006. 200. 10 Jun 2006 <>.

Zitzewitz, Paul W.. Physics Priniples and Problems. Westerville Ohio: Glencoe, 1999.

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