# About the stress acting on the material

I have considered a steel rod of cross section area A and a force of p acting on the two ends along the axis.Now my doubt is whether the internal stress force is p or 2p? My approach is if the length of the rod is l then there Wil be a internal stress of p at each point but when you consider the rod at l/2 we will have 2p internal stress .please some one answer this and help me where I am wrong

• No my approach is let us assume that there are 10 layers of atoms in this length l, now I have applied an equal force in both directions .It means the later next to extreme ends from both ends will have a stress of p, and this stress continues unless and until the two opposite acting forces don't coincide.But in Thai case when the forces reach the 4th and 6th layer both forces act upon a single bond between atoms so that the stress acting on this bond is 2p.So we have a stress of 2p at this point and we take the maximum value as stress for better functioning .Is it correct – mechanics Jul 20 '17 at 13:48
• I'd say that stress is a macroscopic concept (see answer below) and that it the word shouldn't be applied in its technical sense to interactions on an atomic scale. – Philip Wood Jul 20 '17 at 13:54

Suppose the stress is tensile (so the rod is being pulled with equal and opposite forces, +F and –F at either end. Imagine you cut the rod anywhere along it. The forces you'd have to apply to the cut ends to maintain equilibrium would have to be –F and +F. By definition, the tension is F (= |F|) and the tensile stress is $\frac{F}{A}$.