Friction: from kinetic to thermal

The kinetic friction force is the third family, and a special case: it transfers energy from kinetic (ordered) to thermal (disordered). The process is irreversible: once energy is in $E_\text{term}$ it doesn’t spontaneously go back. This irreversibility is what distinguishes friction from all the other forces seen so far, whose arrows could point either way.

Principle — Rule for kinetic friction

The kinetic friction force always does negative work: the arrow exits the $E_\text{cin}$ bubble (of the sliding body) and enters the $E_\text{term}$ bubble (of the body itself or of the surface): $\Delta E_\text{term} = |L_\text{attr}| = F_a\cdot|\Delta\vec{s}|$

Friction transfers energy only one way: from $E_\text{cin}$ to $E_\text{term}$. $E_\text{term}$ is disordered, and cannot spontaneously convert back into kinetic energy.

The thermal energy generated is exactly the absolute value of the work of friction, that is, the friction force multiplied by the distance travelled while sliding. This is why the longer the path, the more the body heats up: friction doesn’t depend only on the endpoints of the path but on the entire route, and it’s precisely this that prevents it from being conservative.

Warning — Static vs kinetic friction

It’s kinetic friction (when surfaces slide against each other) that does negative work on the body. Static friction, on the other hand, does no work if the surface is stationary (like a normal constraint reaction). But if the surface moves — for example the feet of a person walking — static friction can do positive work on the body (this is how we walk: it’s the floor’s static friction that pushes us forward).

Links

Topics: Lavoro ed energia Concepts: Energia termica da attrito · Attrito dinamico · Attrito statico

Related exercises: Problema — Blocco spinto con attrito · Esercizio svolto — blocco su piano inclinato con molla e attrito · Problema — Spingere una cassa in moto