Answer:

In layman's terms, friction is a force that resists one surface from sliding or rolling over another. Therefore, it can be said that friction only occurs when two surfaces are in relative motion, such as when a crankshaft is rotating in a journal bearing or when a ball bearing is rolling along its raceway.

A microscopic view of these surfaces in relative motion reminds us that each surface contains tiny, jagged asperities (rough and uneven surfaces), no matter how closely these surfaces are machined.

Without some form of separation, such as that formed by a lubricant film, these surface asperities may seize upon contact. At the very least, some abrasion, adhesion and/or ploughing of these asperities will take place as the movement occurs.

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