What unit is the friction coefficient expressed in?

The friction coefficient is a dimensionless number that represents the ratio of the force of friction between two bodies and the normal force pressing them together. Since it is a ratio of forces, and both forces are measured in Newtons, the units cancel out. As a result, the friction coefficient does not have any units and is often expressed simply as a numerical value (e.g., 0.5) without any accompanying units.

While "Newton/Meter" suggests a force per unit distance, it does not accurately represent the nature of the friction coefficient, which is purely a ratio. Therefore, the correct understanding is that the friction coefficient, being dimensionless, is not expressed in any of the provided units.