Here's why:
* Friction is a force that opposes motion between surfaces in contact. It's measured in Newtons (N).
* Weight is the force of gravity acting on an object, also measured in Newtons (N).
* Coefficient of friction is a measure of the roughness between two surfaces. It's a ratio of the frictional force to the normal force (which is the force perpendicular to the surfaces). Since both forces are measured in Newtons, the units cancel out, resulting in a unitless number.
The formula for the coefficient of friction (μ) is:
μ = Friction force / Normal force
Since the normal force is equal to the weight of the object in many cases, the formula can be simplified to:
μ = Friction force / Weight
