Friction

For better or for worse, friction is an inescapable force we encounter every moment of our lives. We depend on friction in order to walk, we take advantage of friction in order to light a match, we try to reduce friction in our car engines and door hinges, and friction is generated as the muscle fibers of our hearts contract and relax with each heart beat. Usually physicists and engineers invest a lot of time and energy into trying to reduce or eliminate friction within the moving parts of machinery, but others sometimes look for ways to increase friction. Whether an engineer is trying to design a better set of automobile brakes or a more efficient wind turbine, a thorough understanding of friction is a vital prerequisite.  

Friction occurs whenever two surfaces are in contact with each other, and in general, it is the roughness of the surfaces that determines the amount of friction that results. Even surfaces that look and feel smooth may contain thousands of irregular bumps, pits, ridges, and valleys, although a microscope may be required to actually see them. When two such surfaces slide past one another, the tiny bumps and ridges on one surface can get hung up briefly in the pits and ridges on the other surface. It is the brief locking together of the surface irregularities that creates friction and impedes their motion.

Static friction is the force that must be overcome in order to set a body in motion. Kinetic friction is the force that must be overcome in order to keep a body in motion. Kinetic friction is usually less than static friction, but both types occur mainly because of the surface macro- and microscopic imperfections.

When an object such as a coffee cup is at rest on a table top, some of its surface imperfections are pressed up against the similar imperfections of the table, with the tiny peaks of one surface nestled into the tiny valleys of the other. To set the cup in motion and make it slide across the table, enough force must be applied to get the peaks and valleys on the upper surface up and out of the valleys and peaks on the stationary surface below. Static friction is the force that must be overcome to disengage these peaks and valleys in order for the cup to begin sliding across the table.

https://www.schooltube.com/video/19192f02dc78fcccaf82/

While friction is primarily caused by surface roughness, there are many modern synthetic materials that have exceptionally smooth surfaces. For these materials, the friction that results from surface roughness can be very, very small.

Another way to reduce friction is issuing lubricants. A lubricant is a substance introduced between two surfaces to lessen the friction between them. Common lubricants include:

Oils

Waxes

Grease

Ice

Friction also can be harmful or helpful. For example all brake systems are based in increase the friction between the wheel and the brake, reducing the force on the wheel and its acceleration. The friction between your foot and the floor allow you to walk and do not felt.

Examples of harmful friction would be an abrasion to the skin from a person sliding on a rug 

Questions

I. What is friction?

II. Friction is an important force in nature. It can be harmful or helpful. Describe some ways it is harmful and some ways it is helpful.

III. Describe a situation in which using wheels would reduce friction between a moving object and the surface over which it travels.

IV. Hypothesize what your life would be like if there were no friction. Which actions would be more difficult? Which would be easier?

V. How lubricants affect friction between two objects?

VI. Give three daily life examples of lubricants reducing friction between two objects.