# What does d'Alembert's principle actually tell us?

I have read in my book that d'Alembert's principle states:

The reversed effective forces and the impressed forces at any point of a system are always in equilibrium.

Now when I opt to study Lagrangian mechanics I have seen a new version of d'Alembert's principle in the light of virtual work. I hardly find the similarity between these two. I cannot relate them in anyway.

• More on d'Alembert's principle: physics.stackexchange.com/q/82884/2451 – Qmechanic Sep 25 '16 at 17:48
• @Qmechanic I don't find my answer till now.My question Can we consider 'The virtual work done by applied forces and virtual work done by the reversed effective forces are always in equilibrium at each point of the system' as d'Alembert's principle rather than the version of this principle we are familiar with.If the answer is 'yes' then how can be shown these two principles are equivalent. – Arnab Chatterjee. Sep 25 '16 at 18:09
• $\uparrow$ Which book? Which page? – Qmechanic Sep 25 '16 at 18:15
• It is ''Dynamics of Rigid Bodies'' written by Brahma Nand , B.S. Tyagi and Bhu Dev Sharma of Kedar Nath Ram Nath Publication. – Arnab Chatterjee. Sep 25 '16 at 18:24