Coefficient of friction (COF) refers to the strength of two surfaces in movement against each other. This coefficient is used to express how much friction will occur when two surfaces are in contact and to evaluate the slipperiness of these surfaces. There are two types of COF:Static COF: It refers to the COF before the two surfaces have started to move. This COF indicates how much friction is created by the force applied between the two surfaces in static state. The static COF amounts to the sliding or moving threshold of an object.Dynamic COF: It refers to the COF after the two surfaces have started to move or are moving continuously. It represents the conditions of the two surfaces’ being in continuous movement against each other. It can differ from the static COF.