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Hello Sean
Can we have your PDF of your note as well as ressources ?
thank you 
Hey @emanuel_placinta,
I’m afraid I don’t circulate (or retain) these handwritten notes. They tend to be quite off the cuff - it’s much better that you recreate your own version with you own additions and emphases as you move through the course.
Seán
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Hi Dr Sean.
In this example we can see that the quantites ux1 and ux2 are constants(nodal displaments), If this quantites are constants, which means its derivate is zero, therefore, in the last lecture when you derive the general expression of potential energy(V(UE)) with respect to each displacement( UE vector) to equote to zero, you do not take this UE vector as a constant too?
Hi @edwinhenaove, in the context of the derivation of the element stiffness matrix; we have an expression for the potential energy in terms of nodal displacements. The potential energy is therefore a function of the nodal displacements which are not constant, but actually represent all possible configurations of nodal displacements. This is why we set the derivative to zero after differentiating w.r.t. nodal displacement - we seek the minimum, i.e. the displacement configuration that corresponds to minimum potential energy.