I'm studying FEM. My book says that "the work of internal forces is equal the the work done by external forces" and it states it in an equation Wi=We.
But then it somehow starts discussing potential energy of external and internal forces, and somehow it turns out their sum is not 0. I'm having trouble understanding this. Later in the book it goes on to say that "an element is in the state of balance when the first variation of the total potential energy is equal to 0".
So my question is why isn't the potential energy from external forces equal to the potential energy of internal forces? Or in other way, why isn't their sum equal to zero?
Just to confirm I am correct about it I did some research and found a paper online with an example which indeed confirms that the potential energy isn't zero: