Friction Explained
What friction is, static vs kinetic friction, the formula F = μN, coefficients of friction, how friction helps and hurts, and examples from brakes to ice.
Key takeaways
- Friction opposes motion between surfaces in contact.
- Static friction stops things starting to move; kinetic friction acts once they slide.
- Friction force ≈ μ × normal force, where μ is the coefficient of friction.
On this page
Friction is the force that resists surfaces sliding over each other. Without it you couldn’t walk, drive or hold a pen; with too much, machines wear out and waste energy.
Static vs kinetic friction
- Static friction acts before motion starts; it’s why a heavy box won’t move until you push hard enough.
- Kinetic (sliding) friction acts once surfaces slide, and is usually smaller than the maximum static friction.
The formula
F = μ × N
- μ = coefficient of friction (no units)
- N = normal force, the push between the surfaces (on flat ground, N = mg)
| Surfaces | Approximate μ (kinetic) |
|---|---|
| Rubber on dry concrete | High (around 0.7 or more) |
| Rubber on wet road | Lower |
| Steel on steel (lubricated) | Low |
| Ice on ice | Very low |
Worked example
A 20 kg box on a floor with μ = 0.3: N = 20 × 9.81 ≈ 196 N, so friction ≈ 0.3 × 196 ≈ 59 N. You need more than about 59 N to keep it sliding at constant speed.
When friction helps
Walking, tyre grip, brakes, and holding objects. Cornering cars rely on friction for centripetal force.
When friction hurts
Wear, heat and wasted energy in engines and machines. Lubricants, bearings and smooth surfaces reduce it; see work, energy and power for efficiency.
Air resistance
Air resistance is a kind of friction with the air; it’s why falling objects reach terminal velocity; see the free fall calculator.
Resolving forces on a slope is explained in vectors and scalars.
Frequently asked questions
Does friction depend on surface area?
For many everyday situations, friction depends mainly on the materials and normal force, not contact area.
Why is ice slippery?
A thin layer of water or disordered molecules on the surface greatly reduces friction.
Is friction a contact force?
Yes. It acts where surfaces touch.
Sources
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