Definition of Coefficient of Friction
The coefficient of friction (μ) is a dimensionless scalar quantity that quantifies the amount of friction between two surfaces. It is defined as the ratio of the frictional force (Ff) that opposes motion to the normal force (Fn) pressing the surfaces together, expressed by the formula μ = Ff / Fn.
Types: Static vs. Kinetic Friction
There are two primary types: the static coefficient of friction (μs), which applies when surfaces are at rest relative to each other and resist initial motion, and the kinetic coefficient of friction (μk), which applies when surfaces are already sliding past each other. Typically, the static coefficient is greater than the kinetic coefficient, meaning it takes more force to start an object moving than to keep it moving.
Practical Example and Calculation
Imagine pushing a 10 kg box (approximately 98 N normal force on Earth) across a floor. If it requires 49 N of force to keep the box sliding at a constant velocity, the kinetic frictional force is 49 N. Using the formula, the kinetic coefficient of friction (μk) would be 49 N / 98 N = 0.5. This value indicates how 'slippery' or 'grippy' the surfaces are relative to each other.
Factors Affecting the Coefficient
The coefficient of friction is primarily dependent on the nature and roughness of the two materials in contact. It is largely independent of the apparent contact area between the surfaces or the magnitude of the normal force, for typical engineering applications. Surface contaminants like dirt or lubricants can also significantly alter its value.