Friction Force Calculator
Friction (N)
30
How it works
Friction force is the resistance to relative motion between two surfaces in contact: F_friction = μ × F_normal, where μ is the coefficient of friction and F_normal is the perpendicular (normal) force pressing the surfaces together.
**Static vs. kinetic friction** Static friction prevents surfaces from sliding — it adjusts up to a maximum value of μ_s × F_normal. Once motion begins, kinetic (sliding) friction takes over: F_k = μ_k × F_normal. Kinetic friction is typically 10–30% less than maximum static friction. This is why it's harder to start pushing a heavy box than to keep it moving.
**Coefficient of friction values** Dry steel on steel: μ_s ≈ 0.74, μ_k ≈ 0.57. Rubber on dry concrete: μ_s ≈ 0.7–0.8 (why tires grip pavement). Ice on ice: μ_k ≈ 0.03 (why skating is possible). PTFE (Teflon) on steel: μ ≈ 0.04. Wet surfaces significantly reduce μ — braking distance on wet roads roughly doubles compared to dry.
**Rolling friction** Rolling friction (or rolling resistance) is much smaller than sliding friction — typically μ_rolling = 0.001–0.01 for steel on steel (railroad wheels). This is why wheels are fundamental to transportation — they convert sliding friction to rolling friction.
**Angle of friction and self-locking** The angle of friction φ = arctan(μ). A surface inclined at angle α: if α < φ, a block on the surface won't slide without applied force (self-locking). If α > φ, it slides. Screws and wedges use this principle — their thread angle is less than the friction angle to maintain position without applied torque.
Frequently Asked Questions
- For sliding: check if horizontal force F > μ × W (weight). For tipping: check if the overturning moment (F × h) > restoring moment (W × b/2), where h is height of force application and b is base width. Whichever limit is reached first determines the failure mode. Wide, low-profile objects tend to tip; tall, narrow objects tend to slide. The transition: at μ = b/(2h), both occur simultaneously. For furniture stability, placing heavy objects low and using anti-tip straps prevents tip-over from lateral loads (curious children, earthquakes).
- Rubber on dry concrete: 0.7–0.8 (why cars grip pavement). Rubber on wet concrete: 0.4–0.5 (why wet roads are dangerous). Steel on steel (dry): 0.6–0.7. Steel on ice: 0.03–0.05. Wood on wood (dry): 0.3–0.5. Leather on metal: 0.4–0.6. PTFE (Teflon) on steel: 0.04–0.1 (lowest of any solid materials). Synovial fluid in joints: 0.001–0.003 (lower than Teflon). Values vary significantly with surface roughness, temperature, contamination, and relative velocity — these are approximate reference values.
- Amontons' Law states F_friction = μ × F_normal, with no area term. Microscopic explanation: real surfaces contact only at asperities (tiny peaks), not across the whole apparent area. Actual contact area is proportional to normal force regardless of apparent area — a larger surface has more asperities but each carries less load. This holds well for hard, dry surfaces (metal-metal, wood-wood). Exceptions: soft materials (rubber on pavement — contact area matters for traction), lubricated surfaces (hydrodynamic effects depend on area), and adhesion-dominated contacts (gecko feet, microelectronics bonding).
- Brake squeal is friction-induced vibration — a stick-slip phenomenon. At certain speeds and pressures, friction alternates between static (high) and kinetic (lower), causing the rotor/pad system to oscillate. The frequency depends on rotor/caliper stiffness and mass. Solutions: chamfered or slotted brake pads (disrupt oscillation), shims (damping), different pad compounds (higher or lower μ, different stick-slip behavior), bedding-in (transfer thin uniform pad material to rotor surface). ABS systems actually use controlled slip to stay near maximum friction — brief lock-up uses static friction until slip converts to kinetic.