Question

What causes friction between two surfaces in contact?

Answer 1

The roughness of surfaces causes friction between two surfaces in contact. To ordinary eyes, the surfaces may seem smooth, but at a microscopic level, the surfaces have crest and trough. Under magnification, they look a little like mountain ranges, full of peaks and valleys. Hence, when two surfaces come in contact, the motion between them is reduced because of the crest and trough on the surfaces.