Why does friction occurs when two surfaces are in contact?  

Expert Answers

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Friction, in simplest terms, is a force that opposes motion. It occurs when two surfaces are in contact with each other. Friction occurs due to the roughness of the surfaces in contact. Even if a surface appears really smooth to our eyes, it is not so at micro scale or nano scale. A microscope can reveal the roughness features on a surface. Friction develops due to these roughness features or elements. This is the reason why a book stops after sliding for some distance on a perfectly smooth looking glass on top of a table. 

Friction between solid surfaces can be overcome by reducing the roughness of the surfaces. One common method to do that is by lubrication. Lubricants reduce the roughness and thereby, the friction between the two surfaces. That is why we put lubricant in door joints, car engine, etc.

Hope this helps.

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