To help make my question clearer, I will be referencing equations from the following paper. http://www.engr.colostate.edu/~thompson/hPage/CourseMat/Tutorials/Solid_Mechanics/energy.pdf Before I am informed, I have read all articles on this site relating to this theorem and they are not clear to me. As such, if you choose to post, please to not reference or quote any text from those previous threads. "This theorem states that the strain energy of a linear elastic body (structure), in static equilibrium, under the action of constant surface tractions and body forces, is equal to half the work these tractions and body forces would perform in moving through their respective displacements." Pg 14 The right hand side of the equation is clearly the external work of the system. Since the External Work + Internal Work = 0, then that means the left hand side should equal the negative of the internal work. So, Internal Work = -2(Strain Energy)??? Neglect heat, and other energy, I thought the internal work was equal to strain energy. Can someone explain this? Any help would be appreciated. Thank you.