# Hooke's law and elongation

How to calculate the elongation when we know we all other parameters?

Two sides of a rod is pulled by two forces of F newtons in opposite directions. What should we consider F or 2F in the equation? I used 2F and got wrong answer as per the book. But, if it is true, why should be use F only? If the forces are unequal which force should be use to calculate the elongation?

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olgranpappy
Homework Helper
if the forces are unequal the rod will accelerate.

You are right, I missed the point, it was my blind spot. Thanks so much for making me realise that. Should we use 2F or F to calculate the elongation?

alphysicist
Homework Helper
Hi manjuvenamma,

We use F is there is a force F on both sides. And if it is being stretched there will have to be a force pulling on both sides.

In your head,, think of taking a cut through the material. At the cut, draw in the forces on each surface necessary for each of the two pieces to remain in equilibrium (no acceleration). The forces that you have drawn are the forces experienced by the material at the location of the cut. Are these forces F or 2F? Do these forces depend on the location of your cut?