### Revision for “Coefficients of Kinetic and Static Friction” created on October 19, 2021 @ 04:43:07

Title | Coefficients of Kinetic and Static Friction |
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Content | <a href="http://www.tribonet.org/wiki/friction/" target="_blank" rel="noopener">Friction</a> is a force resisting relative motion and it occurs at the interface between the bodies, but also within the bodies, like in case of fluids. The concept of <a href="http://www.tribonet.org/wiki/coefficient-of-friction/" target="_blank" rel="noopener">friction coefficient</a> was first formulated by <a href="http://www.tribonet.org/who-is-the-first-leonardo-da-vinci/" target="_blank" rel="noopener">Leonardo da Vinci</a> and is defined according to the following equation:
<p style="text-align: justify;">[math]
\begin{eqnarray}
\label{complete_sys1}</p>
<p style="text-align: justify;">\mu = F_f/F_N</p>
<p style="text-align: justify;">\end{eqnarray} [/math]</p>
<p style="text-align: justify;">where [math]F_f[/math] is the friction force and [math]F_N[/math] is the applied normal load. The magnitude of the coefficient of friction is determined by the properties of the surfaces, surroundings, surface features, presence of the lubricant, etc. A list of typical friction coefficients can be found <a href="http://www.tribonet.org/wiki/friction-coefficients-in-atmosphere-and-vacuum/" target="_blank" rel="noopener">here</a>.</p>
<h2 style="text-align: justify;">Definition and Formula</h2>
Typically friction can be distinguished into static friction and kinetic friction. Static friction force [math]F_{fs}[/math] is the friction force prior the initiation of motion. Kinetic friction (also known as dynamic, or sliding) force [math]F_{fk}[/math] is the friction force developed during the motion. It should be noted that these forces are different with a following relation holding in most situations:
<p style="text-align: justify;">[math]
\begin{eqnarray}
\label{complete_sys1}</p>
<p style="text-align: justify;">F_{fk}<=F_{fs}</p>
<p style="text-align: justify;">\end{eqnarray} [/math]</p>
Correspondingly, there are two friction coefficients, static [math]\mu _s[/math] and kinetic friction [math]\mu _k[/math] coefficients.
Static friction coefficient is calculated according to the following equation:
<p style="text-align: justify;">[math]
\begin{eqnarray}
\label{complete_sys1}</p>
<p style="text-align: justify;">\mu _s = F_{fs}/F_N</p>
<p style="text-align: justify;">\end{eqnarray} [/math]</p>
<p style="text-align: justify;">Kinetic friction coefficient is calculated according to following equation:</p>
<p style="text-align: justify;">[math]
\begin{eqnarray}
\label{complete_sys1}</p>
<p style="text-align: justify;">\mu _k = F_{fk}/F_N</p>
<p style="text-align: justify;">\end{eqnarray} [/math]</p>
See this introduction video regarding the static and kinetic friction forces:
[embed]https://www.youtube.com/watch?v=aZiVixJBNfE[/embed] |

Excerpt |