1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Dec 19, 2009 #1
    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 ????????
     
  2. jcsd
  3. Dec 19, 2009 #2
    Re: stress

    Could you rephrase your question? It's hard to make any sense of it in it's current form.
     
  4. Dec 19, 2009 #3
    Re: stress

    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.
     
  5. Dec 19, 2009 #4

    FredGarvin

    User Avatar
    Science Advisor

    Re: stress

    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?

     
  6. Dec 19, 2009 #5
    Re: stress

    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.
     
  7. Dec 19, 2009 #6

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    Re: stress

    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).
     
  8. Dec 19, 2009 #7

    FredGarvin

    User Avatar
    Science Advisor

    Re: stress

    Excuse me????
     
  9. Dec 20, 2009 #8

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    Re: stress

    Equilibrium is obtained only for a very specific combination of forces and torques; unbalanced forces exist, as does stress.
     
  10. Dec 21, 2009 #9
    Re: stress

    Error!
     
    Last edited: Dec 21, 2009
  11. Dec 21, 2009 #10

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    Re: stress

    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).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Is there no stress when a net force is not equal to 0
Loading...