Friction: Kinetic and Static

What is kinetic and static friction?

Kinetic friction is the force that a surface applies to an overlying, moving object and opposes the applied force/motion.

Static friction is the force that a surface applies to an overlying, non-moving object and must be overcome to move the object.

The magnitude of kinetic friction is relatively constant and the magnitude of static friction is variable. For example, if you want to pull an airplane with a rope, before you pull, the static force is zero. As you begin pulling the static friction increases as you exert yourself more and more to move the still, immobile plane. Once the plane begins to move, kinetic friction takes over. Commonly, the force to keep an object moving - especially a heavy one - is less than the force to move it in the first place.

How are kinetic and static friction calculated?

First off, the relationship between the force of friction ($F_{fr}$) is based on the characteristics of the two objects that are in contact with each other. Further, the $F_{fr}$ is perpendicular to the $F_{N}$ and can be calculated using a predetermined coefficiant ($\mu$):

Table 60.1: Kinetic and Static Friction.

Kinetic friction	$F_{fr} = \left(\mu_{kinetic}\right)\left(F_{N}\right)$
Static friction	$F_{fr} = \left(\mu_{static}\right)\left(F_{N}\right)$

Note: $F_{Kinetic \ fr.}$ is usually less than $F_{Static \ fr.}$ and $\mu$ is directly related to the amount of friction between the two objects, i.e. the more friction, the higher the $\mu$, the higher the $F_{fr}$ and the more closely it resembles $F_{N}$.