Why does my calculation give zero stress when stretching a rod?

[ajayguhan]

I know that I'm wrong but couldn't find where I'm

Stress=internal force /area

When the body is in equilibrium, force applied equals internal force.therefore stress=force applied/area
Let us stretch a rod by two equal and opposite force.,say f and -f...thus total force is zero

Stress= force applied/area
Stress=0 , since total force applied is zero

When we stretch a rod, there is a stress. How come it came zero for me...?



Thanks in advance.

 

[PhanthomJay]

The net external force acting on the rod is zero, therefore, the rod is in equilibrium and is not moving (or moving at constant velocity), in accordance with Newton's First Law.

Now draw a Free Body Diagram of a piece of the rod, by pictorially cutting it say down the middle and examining the force acting on the cut end. Since the piece of the rod is also not moving, the net force on that cut piece must also be zero, and thus the force on the cut end must also be F, again in accordance with Newton's first law. This is an internal force, producing an internal stress F/A on that cut section, or any section of the rod.