Coefficients of Kinetic and Static Friction

Coefficients of Kinetic and Static Friction

<h2>What is friction?</h2> <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> <lite-youtube videoid="cKadmarF6rM"></lite-youtube> <h2 style="text-align: justify">Static vs Kinetic Friction</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}&lt;=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: <lite-youtube videoid="aZiVixJBNfE"></lite-youtube>