Kinetic Friction

Kinetic friction, abbreviated as \(f_k\), also called dynamic friction or sliding friction, is the force that opposes the relative motion of two surfaces that are moving with respect to each other.  When an object is in motion, the irregularities on the surfaces in contact slide past each other, creating a resistance to motion.  The strength of kinetic friction is generally less than the strength of static friction, but it depends on the materials in contact, the force pressing the two surfaces together, and the roughness of the surfaces.  Kinetic friction is also affected by the speed of the moving object, it typically increases as the speed of the object increases.

Laws of Kinetic Friction

The force of kinetic friction is proportional to the normal force  -  The force of kinetic friction is directly proportional to the normal force pressing the two surfaces together.
The force of kinetic friction is independent of the velocity of the object  -  Unlike air resistance, which increases with the velocity of an object, the force of kinetic friction is generally independent of the velocity of the object.  This means that the force of kinetic friction remains constant as long as the normal force and coefficient of kinetic friction remain constant.
The force of kinetic friction is independent of the contact area between the surfaces  -  The force of kinetic friction is also independent of the contact area between the surfaces.  This means that the force of kinetic friction will be the same whether the surfaces are in point contact or in full contact.

It's important to note that the laws of kinetic friction assume that the two surfaces in contact are smooth and clean.  In real world situations, surfaces may have imperfections, be covered in debris, or have surface coatings that affect the coefficient of kinetic friction.

 

Kinetic Friction formula
\( f_k \;=\; \mu_k \; F_n \)     (Kinetic Friction) \( \mu_k \;=\; f_k \;/\; F_n \) \( F_n \;=\; f_k \;/\; \mu_k \)
Symbol	English	Metric
\( f_k \) = Kinetic Friction	\( lbf \)	\(N\)
\( \mu_k \) (Greek symbol mu) = Kinetic Friction Coefficient	\( dimensionless \)	\( dimensionless \)
\( F_n \) = Normal Force	\( lbf \)	\(N\)