Friction Force Calculator

f = μ × N. Force resisting motion between contacting surfaces.

Inputs

Rubber on dry asphalt: ~0.7-0.9. Steel on steel: ~0.1-0.6. Ice: ~0.02-0.1.

Result

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General calculation reads

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How to use this calculator

  • Enter μ for the surfaces in contact.
  • Enter normal force.
  • Pick static or kinetic.

About this calculator

Friction force = coefficient × normal force. Coefficient depends on material pair: rubber on dry asphalt 0.7-0.9, rubber on wet asphalt 0.4-0.6, steel on steel 0.1-0.6, ice on ice 0.02-0.1. Static friction (μ_s) prevents starting motion; kinetic (μ_k) opposes ongoing motion. Almost always μ_s > μ_k — this is why a stuck box suddenly slides easily once it breaks free.

Frequently asked

Where do μ values come from?+
Empirical — measured for material pairs. Hyperphysics, NIST tables, or engineering handbooks list standard values.
Why μ_s > μ_k?+
Microscopic surface bonding takes time to break. Once moving, surfaces glide before fully re-bonding.
Tire grip on dry road?+
μ ≈ 0.7-0.9 for performance tires. Wet: 0.4-0.6. Snow: 0.2. Ice: 0.05-0.1. Rolling friction is smaller still (μ_rolling ≈ 0.01-0.02).
Why no μ × area?+
Friction is independent of contact area for rigid surfaces — surprising but standard model. Tire performance includes effects this simple model doesn't capture.
Coulomb model limits?+
Breaks down for soft tires, lubricated surfaces, or extremely high pressure. For most rigid-body engineering, μN suffices.

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