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How It Works
Friction is the force that resists sliding between two surfaces in contact. The friction equation f = μ × Fnormal multiplies a dimensionless coefficient (μ) by the normal force pressing the surfaces together. Static friction keeps an object from starting to move; kinetic friction acts once it is already sliding.
The coefficient depends on the material pair and surface conditions. Static coefficients are typically higher than kinetic ones, which is why it takes more force to start pushing a box than to keep it sliding.
Example Problem
A 50 kg crate sits on a concrete floor (μkinetic = 0.4). How much kinetic friction force acts on it while sliding?
- Normal force on a flat surface: Fnormal = mg = 50 × 9.81 = 490.5 N
- f = 0.4 × 490.5 = 196.2 N
Frequently Asked Questions
What is the difference between static and kinetic friction?
Static friction prevents an object at rest from starting to move and can vary from zero up to a maximum value. Kinetic friction acts on objects already in motion and stays roughly constant. The static coefficient is usually higher than the kinetic one.
How do you find the coefficient of friction?
Divide the measured friction force by the normal force: μ = f / Fnormal. For a 20 N friction force and a 100 N normal force, μ = 0.2.
Does friction depend on surface area?
For most dry surfaces, friction is independent of the contact area. It depends primarily on the normal force and the materials involved. This is one of the classical laws of friction (Amontons' laws).
What is a normal force?
The normal force is the perpendicular force a surface exerts on an object. On a flat horizontal surface it equals the object's weight (mg). On an incline it equals mg cos(θ).
Related Calculators
- Force Equation Calculator — calculate force, mass, or acceleration using F = ma.
- Newton's Second Law Calculator — explore additional forms of F = ma.
- Weight Equation Calculator — find the gravitational force (weight) used to compute normal force.
- Gravity Equations Calculator — calculate gravitational acceleration used in normal force.
- Kinetic Energy Calculator — find the energy lost or gained due to friction forces.
- Force Unit Converter — convert between newtons, pounds-force, and other force units.
Reference: Tipler, Paul A. 1995. Physics For Scientists and Engineers. Worth Publishers. 3rd ed.