Friction Force Calculator

How it is calculated

The tool applies the Coulomb friction model for an object resting on a flat horizontal surface. First the normal force is taken as the weight: N = m × 9.80665, with mass m in kilograms and N in newtons. Then the friction force is F = μ × N, where μ is the dimensionless coefficient of friction. Both results are shown in newtons, rounded to two decimals.

The mass must be above zero and μ cannot be negative; μ = 0 simply gives zero friction. There is no field for the normal force itself, only for mass, and no angle: on a slope the normal force is smaller than the weight, so this result would be too high. The tool does not know whether μ is static or kinetic, so the number you enter decides whether F is the force to start motion or to keep it going.

Common mistakes

• Entering the weight in newtons in the mass field. The field is in kilograms; a 245 N weight must be typed as about 25 kg, otherwise the answer comes out about ten times too high.
• Using a static coefficient when the object is already sliding, or the other way around. Kinetic μ is normally lower than static μ, so the force that keeps it moving is smaller than the force to start it.
• Applying the result to an object on a ramp. The tool assumes a flat surface; on an incline, N = m·g·cos(angle), which is smaller.
• Typing μ as a percentage (40 for 0.4). It is a plain ratio, and 40 would give friction one hundred times too large.
• Expecting contact area to matter. In this model it does not; only μ and the normal force enter, so a wider block of the same mass gives the same friction.