Why the coefficient of friction cannot be zero?

Answer 1

The coefficient of friction is an empirical measurement and has to be measured experimentally, and cannot be found through calculations. Rougher surfaces tend to have higher effective values. Most dry materials in combination have friction coefficient values between 0.3 and 0.6. Values outside this range are rarer, but Teflon, for example, can have a coefficient as low as 0.04. Therefore, a value of zero would mean no friction at all, an ELUSIVE property even magnetic levitation vehicles have drag.

Answer 2

The coefficient of friction cannot be zero because some level of interaction between two surfaces is always present, even in seemingly smooth surfaces. Friction arises from surface roughness and intermolecular forces that resist motion, so a non-zero coefficient is necessary to account for this resistance.