When the applied force exceeds the maximum value that static friction can oppose, the body begins to slide and the friction changes nature: it becomes kinetic. Unlike the static case, kinetic friction has a well-defined value, proportional to the normal constraint reaction through the coefficient of kinetic friction .
Key formula
Kinetic friction is always directed opposite to the motion of the body relative to the surface.
Since it is always opposed to the displacement, kinetic friction always does negative work. Over a displacement , the dissipated work is
This work does not vanish: it represents kinetic energy that is taken from the motion and converted into thermal energy, that is, into heat that warms the body and the surface. It is kinetic friction that makes our hands warm up when we rub them, or a car’s brakes when braking. The ordered mechanical energy of motion is thus transformed into disordered thermal agitation, and this transformation is not reversible: the body does not spontaneously set off again by reabsorbing the heat produced.
Warning
Be careful not to confuse the two types of friction from an energy standpoint. Static friction does not do work if the surface is stationary: the contact point does not move, so there is no work. It can, however, do work if the surface moves. This is the case of a person walking: it is precisely the static friction between the foot and the floor that pushes forward and accelerates the person, without dissipating energy as heat. Kinetic friction, on the other hand, always dissipates.
Links
Topics: Attrito Concepts: Attrito dinamico · Energia termica da attrito
Related exercises: Problema — Blocco spinto con attrito · Esercizio svolto — blocco su piano inclinato con molla e attrito · Problema — Spingere una cassa in moto