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Question
what is the origin of the normal reaction force and friction?
Frictional force f is given by f=mu (normal reaction)
mu is coefficient of friction
is this just an empirical relation ? Are normal reaction and friction really related in terms of their causes?

Answer
Both the normal force and the frictional forces are the result of interactions between the electrons of the two contact surfaces. The frictional force between two surfaces is due to both the magnitude of the force pressing the surfaces together and the nature of the surfaces in contact. The stronger the force pressing the surfaces together, the greater the resulting frictional force. Interestingly, both very rough surfaces (for example, sandpaper) and very smooth surfaces (glass) tend to have very high coefficients.

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James J. Kovalcin

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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