What is the formula for the coefficient of friction?

The coefficient of friction is a measure of how much frictional force exists between two surfaces in contact relative to the normal force pressing them together. The correct formulation for the coefficient of friction can be described as the ratio of the frictional force to the normal force acting on the object.

Thus, the coefficient of friction is calculated by taking the friction force and dividing it by the normal reaction force (the perpendicular force acting between the surfaces). This relationship helps to quantify how easily one surface can slide over another. When the friction force increases, so does the coefficient of friction, indicating a greater resistance to motion due to the surfaces' characteristics.

The other choices do not correctly represent the relationship between friction force and normal force. For instance, suggesting that the friction coefficient equals the friction force on normal reaction or using weight divided by surface area does not adhere to the definition, which explicitly requires a relationship between force and the normal force. Therefore, the chosen answer aligns with the fundamental principles of friction mechanics.