Difference in Static & Kinetic Friction

By Chuck Cox
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Friction is one of the many everyday physical phenomena that we tend to overlook and even take for granted. When we think about friction, it is typically viewed in negative terms. For instance, the average automobile expends about 20 percent of its engine power just to overcome the forces of friction. Whenever we do physical work, we usually have to overcome frictional forces. However, friction has some positive aspects, too. For starters, we wouldn't be able to walk without friction, it would be impossible to write without friction, and many buildings and structures would not stand in the absence of friction.

The Two Types of Friction

There are two basic types of friction: static and kinetic. Friction is treated as a force by mathematicians and physicists and is applied in Issac Newton's second law, in which force is equal to mass times acceleration.

Static friction is the frictional force applied to bodies at rest. An example of this is when we place a cinder block on a slanted roof and the block remains at rest. Static friction is what keeps it from sliding off the roof.

Kinetic friction is the frictional force that works against bodies in motion. If we push the cinder block up the the roof, we have to exert a force that is strong enough to overcome both the frictional force and the cinder block's weight.

Static Friction

Static friction is actually a relationship between two surfaces that are in contact and at rest. The maximum static frictional force of two objects in contact is equal to the smallest amount of force required to start motion between them. Static friction is defined mathematically as:

F(s) is less than or equal to u * N

Where F(s) = static frictional force, u = coefficient of friction, and N = force required to create motion.

The coefficient of friction is a value that is less than 1 and is a function of the surface properties. Rougher surfaces would have a lower coefficient value, whereas smooth or lubricated surfaces would have higher values.

Kinetic Friction

Just like with static friction, kinetic friction is also a relationship between the two surfaces of the objects involved. The kinetic frictional force of the two objects that are in relative motion is mathematically defined as:

F(k) = u * N

Where F(k) = kinetic frictional force, u = coefficient of friction, and N = force required to create motion.

The coefficient of friction is a value that is less than 1 and is a function of the surface properties. Rougher surfaces would have a lower coefficient value, whereas smooth or lubricated surfaces would have higher values.

Frictional Forces In Everyday Life

An assessment of frictional forces would not be complete without examining some of the instances where it occurs in our world. One good example of static forces in action pertains to beach houses that we see sitting on stilts. Most of these stilts are held in place by the static frictional force between the wet sand and the stilt.

If we were to simultaneously drop a marble and a feather from the Empire State Building, our instincts tell us that the marble would hit the ground first. However, Newton's Laws tell us that they should land at the same time. The reason they won't is because of friction with the air, and the fact they have different coefficients of kinetic friction.

About the Author

Chuck Cox started a career as a technical writer in 1992. As a ghost writer, wrote math/statistic lesson plans, and co-authored book on hedge funds through eLance.com. He also has several documents published on eHow. Cox holds a Bachelor of Science in physics and mathematics from East Tennessee State University.