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Frictional force refers to the result of the surface of one object coming into contact with the surface of another object. When this happens, there is generally some amount of resistance, which is known as friction. Objects are constantly coming into contact with other objects, meaning that this force is quite a common part of daily life.
This type of force can generally be divided into two groups. Static friction results from two objects that are not moving past one another. This principal explains why an object may not slide even if it is on a slope. Kinetic friction occurs when two objects are moving against one another. It can explain the relation between a swimmer and the water she moves through.
The amount of frictional force that results from contact is influenced by a number of factors. The texture of the objects plays an important role in the resulting friction. When two smooth objects, such as hands, are rubbed together, there is minimal friction, but when two rough objects, such as rigid rocks, are rubbed together, there is typically an obvious increase of resistance.
The amount of pressure that is forcing two objects together will also have an impact. If a piece of paper is laying on a table and the wind blows, a small amount of force will result. In the same scenario, if a pen is placed on top of the paper, the resistance will be increased because the pen is pressing the paper to the table.
Frictional force can be reduced by using a range of lubricants, such as oil or grease, because they tend to make contact smoother. Examples can be found in manufactured products, such as vehicle engines, where oil allows the parts to function effectively. Nature also provides examples: a human’s joints are lubricated with synovial fluid that allows comfortable mobility.
In some instances, this force is not only undesirable but also dangerous. When two rough surfaces are rubbed together, heat is usually generated. If this occurs in the vicinity of something flammable, the result could be a fire or explosion.
There are many other instances when friction is useful and products are designed to encourage it. Brakes and tires are prime examples, and these vehicular parts intentionally apply frictional force to make transportation more safe and controllable. The soles of shoes are also often made to exploit this principal as well, or walking on certain surfaces could become quite a challenge.