# How the stress is generated?

1. Oct 27, 2015

### Arceus74

We know that when two equal forces act in opposite direction they nullify each other.
But when two equal forces act on a body yet it deforms eventhough the resultant force is zero?
Can you explain how the stress is created and results in longitudinal strain?

2. Oct 27, 2015

### SteamKing

Staff Emeritus
The net of the two opposite forces is zero, but that doesn't mean these forces disappear individually. All you can infer is that because the net force is zero, the body remains in static equilibrium. Internally within the body, the forces are still acting to pull apart or push together the structure of the material from which the body is composed.

3. Oct 27, 2015

### Arceus74

How are those internal forces generated?

4. Oct 27, 2015

### SteamKing

Staff Emeritus
It depends. The type of load applied to a body and where it is applied on the body influence the nature of the stress.

5. Oct 27, 2015

### Perq

The reaction force IS the stress (somewhat... :D). For system to remain static, material needs to generate force equal to force applied to it, which is exactly stress.
And if you are asking why stress is generated - stress is resistance to outside force to break integrity of the material. You will eventually reach a point, where material can no longer resist the force, and will stop "generating stress", which will make the system non-static, meaning the material simply broke (lost its integrity).

6. Oct 27, 2015

### Staff: Mentor

Per gave the best answer but I'll try to illustrate.

Think of three columns on a tabletop. The first is made of steel, the second of jello, and the third of water. All three experience stress, but they behave differently.

The steel column appears unchanged.

The jello column squats and becomes wider in the middle.

The water column cant resist the stress and collapses, wetting the table.

7. Oct 27, 2015

### Staff: Mentor

The internal forces are generated by stretching or compressing the bonds between molecules. The molecules of the body are attracted to one another. When you apply an extentional stress to the body, you are stretching the bonds like little springs, and this generates the internal forces.

Chet