Hi all, I have a very basic question regarding the topic of stress.While deriving the equations of equilibrium for a plane element,there is a concept called incremental stress.That is,if stress in the x direction for one face is σx the stress in the opposite face is σx + (δσx/δx)*dx. Where the partial derivative represents the rate of change of stress along the length of element and dx is the length of the element in x direction. My question is, why is this considered in the first place ? can anyone please explain this in the case where a bar is under uni axial tension ? Does stress vary along the length of the bar at different cross sections?.I am having problems in understanding this topic and visualizing this. I am learning this subject on my own.So I apologise if I have misstated something. Thanks for going through the post.