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Friction is a force that offers resistance to movement when one body is in contact with another. Friction occurs because no surface is completely smooth. The rough pieces, no matter how small, catch on each other. The rougher the surfaces, the more friction that is present. The amount of friction between two objects depends on how much force is acting in the direction perpendicular to the surface where the two objects are in contact. This perpendicular force is called normal force. For example, if you try to slide or push a book across a table, the amount of friction is dependent on the weight of the book.
In certain situations, friction is desired. Without friction, object in motion would continue their motion indefinitely. Brakes are devices that use friction to their advantage. Without friction, brake pads would not be able to slow down the wheels of a car or bike.
There are instances where minimum friction is desired. One example of desired minimum friction is the moving parts of machines. Excess friction produces heat that in turn can cause expansion of metals that might lock the moving parts of the machine. Lubricants, such as oil, and ball bearings can reduce friction in a machine, allowing it to run more smoothly.
For more information on friction from www.britannica.com: Click Here