Questions and discussion for this lecture live here. Fire away by hitting Reply below 
So in this part, is it posible to say that the element repsented by A’B’ is the element that additional to the translation of the element by the forces in the structure it will also rotate and suffer a deformation. Is that correct?
Hi @Mauricio - A’B’ is a representation of the bar after it has undergone a ‘large’ deformation - by large, we mean a deformation where we can no-longer ignore the geometric non-linearity (additional forces associated with elongation along the deformed axis - rather than the undeformed axis…our simplifying assumption for linear analysis).
The key difference between this analysis and a linear analysis is our recognition that the elongation ‘e’ along the deformed axis is different to ‘u’, the elongation along the undeformed axis.
I’m not sure if I have addressed your uncertainty - please follow up of this hasn’t clarified things for you.
Seán