Coefficients of Kinetic and Static Friction

Manoj Rajankunte Mahadeshwara

April, 14 2017

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:

Manoj Rajankunte Mahadeshwara

I am a postgraduate researcher at the University of Leeds. I have completed my master's degree in the Erasmus Tribos program at the University of Leeds, University of Ljubljana, and University of Coimbra and my bachelor's degree in Mechanical Engineering from VTU in NMIT, India. I am an editor and social networking manager at TriboNet. I have a YouTube channel called Tribo Geek where I upload videos on travel, research life, and topics for master's and PhD students.