Résumé
In the mechanics of systems involving dry frictional contacts it is known that the problem ofevaluating at some instant the contact forces and the accelerations may admit a plurality ofsolutions. Starting from an elementary example of wedging one proposes, as a consistent wayof handling information, to include the contact forces in the description of each state. This infact is what popular time-stepping computation techniques do, as they make reference at eachtime-step to the constraint state calculated in the antecedent one (actually a way of treatingCoulomb law incrementally). The Contact Dynamics technique optionally offers the alternativeof discarding this information, exploring the consequent indeterminations and displaying thesets of solutions as clouds of dots. Examples arising from Granular Mechanics are presented.The so-called Painlev´e paradox is commented. Finally, an explanation of the isostaticity of theequilibrium of a collection of frictionless round grains is proposed.