There are two reasons for the occurrence of the friction force: the surface roughness of the contacting bodies and the mutual attraction of the molecules of the contacting bodies.
The causes of the friction force are as follows: the roughness of the surfaces of the contacting bodies and the mutual attraction of the molecules of these bodies. The first reason is intuitive. But the second should be explained. In the case when the bodies are well polished, upon contact, part of their molecules is very close to each other. Thus, the friction force that appears when the particles of the body come in contact is manifested.
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