Formula: Kinetic (Sliding) Friction Force Sliding friction coefficient
$$F_{\text k} ~=~ \class{blue}{\mu_{\text k}} \, F_{\text N}$$
$$F_{\text k} ~=~ \class{blue}{\mu_{\text k}} \, F_{\text N}$$
$$F_{\text N} ~=~ \frac{ F_{\text k} }{ \class{blue}{\mu_{\text k}} }$$
$$\class{blue}{\mu_{\text k}} ~=~ \frac{ F_{\text k} }{ F_{\text N} }$$
Sliding friction force
$$ F_{\text k} $$ Unit $$ \mathrm{N} = \frac{\mathrm{kg} \, \mathrm{m}}{\mathrm{s}^2} $$
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
$$ F_{\text N} $$ Unit $$ \mathrm{N} = \frac{\mathrm{kg} \, \mathrm{m}}{\mathrm{s}^2} $$
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
$$ \class{blue}{\mu_{\text k}} $$ Unit $$ - $$
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 steel | 0.1 |
| Wood on wood | 0.4 |
| Stone on wood | 0.7 |
| Stone on stone | 0.9 |
From the table you can see that it is most difficult to push, for example, a brick on a stone surface.