What is the main cause of friction?
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Irregularities and interlocking of the two surfaces in contact.
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Heat is produced when two surfaces rub each other.
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When two surfaces come in contact, they exert pressure on each other which causes friction.
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Irregularities on the upper surface only cause friction when two surfaces comes in contact.
Friction arises primarily from microscopic irregularities on both contacting surfaces that interlock and resist sliding. Even apparently smooth surfaces have roughness at the atomic level that creates resistance. Heat (option B) is a RESULT of friction, not a cause. Pressure (option C) affects normal force but not the friction mechanism itself. Option D is incorrect because irregularities on BOTH surfaces contribute to friction.