Is there no stress when a net force is not equal to 0

[phymatter]

is there no stress when a net force is not equal to 0 ,i mean that whenever we talk about stress why is the net force 0 ?

 

[xxChrisxx]

Could you rephrase your question? It's hard to make any sense of it in it's current form.

 

[phymatter]

i mean , that the formulas of stress , strain , young 's modulus applicable to a moving body , because in all the definations in all the books i have seen , they have assumed that the body is in complete equlibrium , ie. net force and torque =0.

 

[FredGarvin]

There has to be a state of equilibrium between all forces. If you look at the infinitesimal element one always sees to illustrate the 18 stresses on a cube's faces (normal and shear stresses), they must be in equilibrium. If you have a net force, what does Newton's second law tell you?

According to the principle of conservation of linear momentum, if the continuum body is in static equilibrium it can be demonstrated that the components of the Cauchy stress tensor in every material point in the body satisfy the equilibrium equations (Cauchy's equations of motion for zero acceleration). At the same time, according to the principle of conservation of angular momentum, equilibrium requires that the summation of moments with respect to an arbitrary point is zero, which leads to the conclusion that the stress tensor is symmetric, thus having only six independent stress components, instead of the original nine.

 

[xxChrisxx]

The above formula's are applicable to any body, you just have to be more careful with one that is movinng. There will always be some friction in a moving object, therefore always a component of the force acting to deform the material, the other is acting to accelerate. If there really was no force opposing motion, then the object would not deform, so stresses would be induced.EDIT: I've just read that back, and it seems potentially confusing.

 

[Andy Resnick]

i mean , that the formulas of stress , strain , young 's modulus applicable to a moving body , because in all the definations in all the books i have seen , they have assumed that the body is in complete equlibrium , ie. net force and torque =0.

Stress is a more general concept than force. Stress can be thought of as a distributed force (a force distributed on a surface, for example).

Because and strain (the generalized version of position) are more general concepts than forces and positions, the mathematics is more complicated, which leads to introductory textbooks making simplifications in order to make the material more appropriate to the course. Equilibrium has nothing to do with stress and strain any more than it does with forces and displacement.

Stresses and couples (the generalization of torque) are written as tensors: not only does the stress vary with where on the object the stress is, but also the direction. For example, if I push a door open, and I stand in the same place, I can still push in many different directions (some directions are more efficient than others in opening the door).

 

[FredGarvin]

Equilibrium has nothing to do with stress and strain any more than it does with forces and displacement.

Excuse me?

 

[Andy Resnick]

Equilibrium is obtained only for a very specific combination of forces and torques; unbalanced forces exist, as does stress.

 

[phymatter]

Equilibrium has nothing to do with stress and strain any more than it does with forces and displacement.

Error!

 

[Andy Resnick]

I'm not sure how to answer: forces act on points, stresses act on surfaces. There certainly can be internal stresses which develop (for a deformable body, for example) under the action of a field of force (the gravitational field is a good example).