Friction Calculator
Calculate static and kinetic friction force, whether an object slides, its acceleration once moving, and its angle of repose — from mass or a direct normal force. Pick a material pair (steel on steel, rubber on concrete, ice on ice…) or enter your own μs/μk. Metric and imperial. Runs entirely in your browser.
Input
Weight is the common case: an object resting on a flat surface. Use direct entry when something else changes the normal force — an incline, an added downward load.
Kilograms in metric mode, pounds in imperial mode. Always used for acceleration once the object is sliding, and for the normal force too when that's set to come from weight.
Newtons in metric mode, lbf in imperial mode.
Output
| Quantity | Value |
|---|---|
| No data yet | |
Guides
Work out whether something slides. Enter a mass (or a normal force directly), pick a material pair, and this tool tells you if the applied force is enough to break static friction — and if so, how fast the object accelerates once it's moving.
It uses classical Coulomb friction:
F_static,max = μs · N — the ceiling static friction can hold before the object starts to slip F_kinetic = μk · N — the resisting force once it's sliding
where N is the normal force pressing the two surfaces together. Everything runs in your browser — no uploads, no sign-up.
How to use it
- Normal force source — leave it on From the object's weight for the ordinary case: an object resting flat on a surface, where N = m·g. Switch to Enter normal force directly for a ramp, an incline, or anything where an extra load changes how hard the surfaces press together.
- Mass — always used for acceleration once the object is sliding (F = ma), even if you entered the normal force directly.
- Material pair — eleven common combinations (rubber on wet/dry concrete, steel on steel dry or lubricated, aluminum on steel, wood on wood, ice on ice, Teflon on Teflon…) with published static/kinetic coefficients. Pick Custom to type your own μs and μk.
- Applied force — the push or pull trying to move the object.
The result tells you immediately whether the object stays put (static friction exactly cancels the push) or slides (kinetic friction takes over, and the leftover net force accelerates it). The breakdown table shows every intermediate force, the resulting acceleration, and the angle of repose — the incline angle at which gravity alone would start this material pair sliding, from tan(θ) = μs.
Why the answer flips at one threshold
Static friction isn't a fixed force — it grows to match whatever's pushing, up to a maximum of μs·N. Below that ceiling, the object doesn't move because friction is exactly canceling the applied force. The instant the applied force exceeds that ceiling, the object breaks loose, and friction drops to the (usually lower) kinetic value μk·N. That's why a heavy object is hardest to start moving and gets easier once it's sliding.
Why is my normal force different from the object's weight?
They're equal only when the object rests on a flat surface with nothing else pushing on it. On an incline, the normal force is the weight's component perpendicular to the surface (mg·cos θ), which is less than the full weight. If something else presses down on the object too, the normal force is higher. Use Enter normal force directly whenever the flat-surface assumption doesn't hold.
What do μs and μk actually mean?
They're the coefficients of static and kinetic friction for a specific pair of materials and surface conditions — dimensionless ratios of friction force to normal force. They're measured, not derived from first principles, and the published values here are typical dry-surface starting points, not a substitute for measuring your actual materials if the answer matters (moisture, surface finish, temperature, and contamination all shift them).
What's the angle of repose for?
It's the incline angle at which an object would start sliding under gravity alone, with no applied push: tan(θ) = μs. It's a quick sanity check on your μs value — a steel-on-steel angle of repose around 36° matches everyday experience; if a "custom" μs gives you an angle near 90°, the coefficient is probably too high.
Related tools
Need the resulting force, mass or torque in different units? The Force Unit Converter, Weight Converter and Torque Converter cover N, lbf, kgf, kg, lb and the common torque units.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/friction-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"unitSystem": "metric",
"normalMode": "weight",
"mass": "10",
"material": "steel-steel-dry",
"appliedForce": "50"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `friction-calculator` tool (Friction Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.