# What is “irreversible displacement”?

In this Wiki page on D'Alembert Principle they say that "The principle does not apply for irreversible displacements, such as sliding friction, and more general specification of the irreversibility is required".

What are the forces that produce "irreversible displacements"? Do all non-conservative forces produce "irreversible displacements" ?

A displacement is said to be reversible with respect to a given direction if it can occur both in that direction and in the opposite direction. Irreversible displacements occur at the boundary of the configuration space. For example, the displacement of a block on a table is reversible in any horizontal direction but is irreversible in the upwards/downwards direction.

Note that this definition does not make any mention to what force produces this displacements. That is because we are actually talking about virtual displacements and those are to be regarded as arbitrary variations on the configuration space. There is some misunderstanding in wiki article when calling sliding friction as an irreversible displacement. Sliding friction is a dissipative constraint force. It is true however that when there is sliding friction the Principle of Virtual Work - which is fundamental for d'Alembert Principle - does not hold, since this kind of force does work along virtual displacements.

• All non-conservative forces do work along virtual displacements, right? – user139621 Jul 10 '17 at 19:35