Formula: Kinetic (Sliding) Friction Force    Sliding friction coefficient

Formula: Kinetic (Sliding) Friction
Kinetic (Sliding) Friction on an Inclined Plane
Sliding friction force acts on a stone

Sliding friction force

Force that acts against the motion of the body being pushed on a rough surface. The magnitude of the sliding friction force depends on both the nature of its own surface and the surface on which it is pushed.

In the experiment, it is found that the sliding friction force is proportional to the normal force \(F_{\text N}\) for many surfaces.

Normal force

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.

Sliding friction coefficient

Sliding friction coefficient here is the proportionality constant between the sliding friction force \(F_{\text k}\) and the normal force \(F_{\text N}\). The coefficient of sliding friction determines how "rough" a surface is. The higher the coefficient of sliding friction, the "rougher" the surface, i.e. the sliding friction force is greater and it is more difficult to move the body on this surface.
Surfaces Sliding friction coefficient \( \class{blue}{\mu_{\text k}} \)
Steel on steel0.1
Wood on wood0.4
Stone on wood0.7
Stone on stone0.9
Sliding friction coefficient for some dry surfaces.

From the table you can see that it is most difficult to push, for example, a brick on a stone surface.

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