Logic status of virtual work laws
I use the term law of virtual work to mean any rule of equilibrium that includes both forces and possible displacements of their points of application. In this chapter I will give a more restricted meaning by referring only to the most (modern) advanced formulations and I will consider their logical status, i.e. whether they are principles of an autonomous mechanics or theorems of ‘another’ mechanics. To demonstrate a law, a proposition, means in the broadest sense, to bring it to laws ‘assumed as known’, with a sense that varies with the epistemological frame of reference. Until the development of modern axiomatic theories and their application to physical science by the neopositivists of the XX century, a principle was considered as acceptable if it had an intuitive nature of evidence, possibly established a priori. Currently there is a more liberal view and one does not require evidence of the principles, only that they must have sufficient strength and do not lead to logical contradictions.