Question

why frictional force decreases when the surface is smooth & increases when the surface is rough?​

Answer 1

Answer:

If the surfaces that are in contact are rough, there is more friction. Hence to reduce friction, the surfaces in contact are kept smooth and polished. It is worth noting that if the two surfaces in contact are of the same metal, the friction increases on polishing the two surfaces.

Answer 2

Answer:

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Friction decreases with smoothness only to a certain degree. When the surfaces are further polished, the Vander Waal forces and electrostatic forces between the atoms of the surfaces increases, this happens because the surfaces come closer when polished.

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