Question

multiple choice question
friction between two surfaces occurs when _______ form(s).
gas
liquid
air gaps
microwelds

Explanation:

Friction between two surfaces occurs due to the formation of microwelds. When two surfaces are in contact, the microscopic irregularities (asperities) on the surfaces come into contact. At these contact points, the high local pressure can cause the surfaces to adhere or form microwelds. To move one surface relative to the other, these microwelds need to be broken, which results in the frictional force. Gas, liquid, and air gaps do not cause friction in the context of solid - solid surface interaction as described here. Gas and liquid can sometimes reduce friction (e.g., lubrication), and air gaps are not relevant to the fundamental mechanism of friction between two solid surfaces.

Answer:

microwelds