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Coefficients of Kinetic and Static Friction

Friction Coefficient
Tribology Wikipedia > Coefficients of Kinetic and Static Friction

What is friction?

Friction 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 friction coefficient was first formulated by Leonardo da Vinci and is defined according to the following equation:

(1)    \begin{eqnarray*} \mu = F_f/F_N \end{eqnarray*}

where F_f is the friction force and F_N 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 here.

Static vs Kinetic Friction

Typically friction can be distinguished into static friction and kinetic friction. Static friction force F_{fs} is the friction force prior the initiation of motion. Kinetic friction (also known as dynamic, or sliding) force F_{fk} 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:

(2)    \begin{eqnarray*} F_{fk}<=F_{fs} \end{eqnarray*}

Correspondingly, there are two friction coefficients, static  \mu _s and kinetic friction  \mu _k coefficients.

Static friction coefficient is calculated according to the following equation:

(3)    \begin{eqnarray*} \mu _s = F_{fs}/F_N \end{eqnarray*}

Kinetic friction coefficient is calculated according to following equation:

(4)    \begin{eqnarray*} \mu _k = F_{fk}/F_N \end{eqnarray*}

See this introduction video regarding the static and kinetic friction forces:

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