# A basic doubt about stress and strain

baldbrain
Homework Statement:
What causes axial deformation in a bar if the force gets balanced out by the internal resistance?
Relevant Equations:
#σ=F/A#
While introducing tensile stress, we're shown a bar fixed at a support being subjected to a force in the axial direction at the opposite end. Then, since the bar is in translational equilibrium, we say that internal forces must balance it out, and this internal resistance per unit area is known as tensile stress. This results in deformation of the bar in the direction of the applied force.
But my question is, if the forces are getting balanced out, what's causing the deformation? ## Answers and Replies

Homework Helper
Homework Statement:: What causes axial deformation in a bar if the force gets balanced out by the internal resistance?
Relevant Equations:: #σ=F/A#

While introducing tensile stress, we're shown a bar fixed at a support being subjected to a force in the axial direction at the opposite end. Then, since the bar is in translational equilibrium, we say that internal forces must balance it out, and this internal resistance per unit area is known as tensile stress. This results in deformation of the bar in the direction of the applied force.
But my question is, if the forces are getting balanced out, what's causing the deformation? The deformation is what happens before the forces get balanced out. And before the vibrations have died out.

Mentor
Homework Statement:: What causes axial deformation in a bar if the force gets balanced out by the internal resistance?
Relevant Equations:: #σ=F/A#

While introducing tensile stress, we're shown a bar fixed at a support being subjected to a force in the axial direction at the opposite end. Then, since the bar is in translational equilibrium, we say that internal forces must balance it out, and this internal resistance per unit area is known as tensile stress. This results in deformation of the bar in the direction of the applied force.
But my question is, if the forces are getting balanced out, what's causing the deformation? First of all, stress is not internal resistance. It is the internal force per unit area.

Secondly, when you stretch a spring, if the forces are balance out, what's causing the spring to deform.

baldbrain
First of all, stress is not internal resistance. It is the internal force per unit area.
Yes, I had said internal resistance per unit area. Do you mean it's not resistive?

baldbrain
Secondly, when you stretch a spring, if the forces are balance out, what's causing the spring to deform.
Yeah, good point. I admit I hadn't thought about that one either. I used to think that the restoring force starts acting only when the applied force stops acting, i.e. after you release the spring.
But it's what @jbriggs444 pointed out, isn't it?

• Delta2
Gold Member
Molecules inside the bar are happy keeping certain natural distance among them.
That keeps an internal balance of forces and distances while the bar is not under any external force; therefore, the bar has and keeps length L (at certain temperature).

Each of those molecules reacts with certain force that proportionally opposes to any external agent trying to increase or decrease that natural distance.
In your case, the external forces (at anchoring point and at end of the bar) cause a small increment of the inter-molecular spaces, which induces a reactive force from each molecule in the chain.

The result is an elonged or deformed bar, that contains internal forces, which summation equals or balance the applied external forces.
The cross area tells you how many molecules on same plane (perpendicular to external forces) are resisting at once.

Tensile stress is inversely proportional to the area of the cross-section of the bar.
The bar breaks at the smallest cross section when the resistive forces of all the molecules on that plane is overwhelmed by the external forces.

Last edited:
baldbrain
Thanks for the insight @Lnewqban
Also thanking @jbriggs444 and @Chestermiller for lighting that bulb in my head  Gold Member
You are welcome. • baldbrain