Friction Calculator

The friction force is calculated using f = μN, where f is the friction force in Newtons, μ (mu) is the dimensionless coefficient of friction, and N is the normal force in Newtons. This formula applies to both static and kinetic friction — only the coefficient value changes between the two types.

Static friction acts on objects that are not moving relative to each other and prevents motion from starting. Kinetic (or dynamic) friction acts when two surfaces are already sliding against each other. The coefficient of static friction (μs) is typically higher than the coefficient of kinetic friction (μk), meaning it takes more force to start an object moving than to keep it moving.

The coefficient of friction is measured experimentally by dividing the measured friction force by the measured normal force (μ = f/N). A common method is to place an object on an inclined plane and gradually increase the angle until the object begins to slide — the tangent of that angle equals the coefficient of static friction.

On an inclined surface at angle θ, the normal force is reduced to N = mg·cos(θ), and the component of gravity pulling the object down the slope is mg·sin(θ). As the angle increases, the normal force decreases while the gravitational component along the slope increases, making objects more likely to slide.

Common values include: rubber on dry concrete (0.6–0.8), wood on wood (0.25–0.5), steel on steel dry (0.4–0.6), steel on steel lubricated (0.05–0.15), ice on ice (0.03–0.1), and PTFE (Teflon) on steel (0.04–0.1). These values vary significantly based on surface condition, temperature, and lubrication.

The SI unit of friction force is the Newton (N), the same as any other force. The coefficient of friction (μ) itself is dimensionless — it has no unit because it represents a ratio of two forces (friction force divided by normal force).

Energy lost to friction (work done by friction) equals the friction force multiplied by the distance over which it acts: W = f × d = μNd. This energy is typically converted to heat. For example, if a 30 N friction force acts over 5 metres, the energy lost is 150 Joules.

The four main types of friction are: (1) Static friction — resistance to starting motion between surfaces at rest; (2) Kinetic (sliding) friction — resistance between surfaces in relative motion; (3) Rolling friction — resistance when an object rolls over a surface, generally much lower than sliding friction; and (4) Fluid friction — resistance due to viscosity in liquids and gases (also called drag or viscous friction).