Static Friction
Static friction is the frictional force that prevents a stationary object from beginning to move when a force is applied to it. It is a self-adjusting force that increases to match the applied force until a maximum threshold is reached; beyond this maximum, the object begins to slide and kinetic friction takes over. The maximum static frictional force is always greater than the kinetic frictional force for the same surfaces, which is why it is harder to start sliding an object than to keep it sliding.
Key Formula
f_s ≤ μs × N
LaTeX: f_s \leq \mu_s N
| Symbol | Meaning | Unit |
|---|---|---|
| f_s | Static frictional force (adjusts up to maximum) | Newton (N) |
| μs | Coefficient of static friction (dimensionless) | dimensionless |
| N | Normal force between the surfaces | Newton (N) |
Worked Example
Problem
A 25 kg box sits on a wooden floor. The coefficient of static friction is 0.45. What is the maximum static friction force before the box starts sliding? Use g = 9.8 m/s².
Solution
Normal force: N = m × g = 25 × 9.8 = 245 N. Maximum static friction: f_s(max) = μs × N = 0.45 × 245 = 110.25 N.
Answer
The maximum static friction force is 110.25 N. Any applied force exceeding this will cause the box to begin sliding.
Static vs Kinetic Friction — Key Differences
| Property | Static Friction | Kinetic Friction |
|---|---|---|
| When it acts | Object is stationary | Object is sliding |
| Magnitude | Variable (0 to f_s max) | Approximately constant |
| Relative magnitude | Higher (μs > μk) | Lower (μk < μs) |
| Formula | f_s ≤ μs × N | f_k = μk × N |
| Example | Book on tilted shelf before sliding | Book sliding across desk |
| Energy conversion | No energy lost (no motion) | Converts kinetic energy to heat |
Interactive Tools
PhET Forces and Motion Basics
Push objects with varying forces to observe static friction holding them in place before motion begins
Open ToolKhan Academy — Static and Kinetic Friction
Detailed video comparing static and kinetic friction with examples
Open ToolDesmos — Friction Exploration
Model the relationship f_s = μs × N graphically for different surface pairs
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Friction
Friction is a contact force that opposes the relative motion or tendency of motion between two surfaces in contact. It arises from microscopic interactions between surface irregularities and is directly proportional to the normal force through the coefficient of friction. Friction is essential in everyday life — it allows us to walk, cars to brake, and objects to remain stationary on inclined surfaces — but it also causes energy loss as heat in machines.
Normal Force
The normal force is the contact force exerted by a surface on an object, acting perpendicular (normal) to the surface at the point of contact. It is a reaction force that prevents objects from passing through solid surfaces and adjusts in magnitude to balance components of other forces. On a flat horizontal surface, the normal force on a stationary object equals its weight; on an inclined surface, it equals the component of weight perpendicular to the slope.
Newton's First Law
Newton's First Law of Motion states that an object at rest remains at rest, and an object in motion continues in motion at constant velocity, unless acted upon by a net external force. This principle is also known as the Law of Inertia and forms the conceptual foundation of classical mechanics. It explains why passengers lurch forward when a bus brakes suddenly, or why a hockey puck slides indefinitely on a frictionless ice surface.
'Static' comes from Greek 'statikos' meaning causing to stand, from 'histanai' (to cause to stand). 'Friction' comes from Latin 'frictio' (rubbing). The distinction between static and kinetic friction was formalised by Charles-Augustin de Coulomb in his 1781 treatise on friction.