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Formula Static Friction Static friction force    Normal force    Static friction coefficient

Formula
Formula: Static Friction
Sliding friction force acts on a stone
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Static friction force

Unit
Force required to set in motion a body resting on a (tilted) rough surface. The amount of static frictional force depends on both the nature of the surface of the body and the surface on which it is placed. For many surfaces, the static friction force is proportional to the normal force \(F_{\text N}\).

The static friction force acts against the external force (traction) applied to the body to make it move. And it is the maximum static friction force that results from the adhesion of two surfaces. This means that if you apply a larger force to the body than the static friction force \(F_{\text R}\), then the body will be set in motion.

Normal force

Unit
Normal force is the force exerted on the body by the surface on which the body is placed. In this way the body does not simply fall through the surface. The normal force acts perpendicular to the surface on which the body is placed.

"Normal" is meant in the geometric sense and means "orthogonal". You could also call the normal force an orthogonal force.

Static friction coefficient

Unit
The static friction coefficient is the constant of proportionality between the static friction force \(F_{\text R}\) and the normal force \(F_{\text N}\). The greater the static friction coefficient, the greater the static friction force and the more difficult it is to get the body moving on this surface.

SurfacesStatic friction coefficient \(\mu_{\text H}\)
Steel on steel0.2
Wood on wood0.5
Stone on wood0.9
Stone on stone1.0
Static friction coefficient for some dry surfaces.