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External vs. internal forces

  1. Feb 20, 2008 #1


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    I am having trouble explaining the difference between internal and external forces, and in fact, even *defining* internal forces.

    Does anyone understand the difference very well?

    My current example is:

    Hold an elastic band loosely between both your index fingers. Now, move your fingers apart. This pull is an external force acting on the elastic. However, the elastic has changed. It's now tight and thinner than it was before. The tension of the elastic is an internal force. This is the *result* of the external force of the pull of your fingers.

    Is this about right?
  2. jcsd
  3. Feb 20, 2008 #2


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    Draw a surface just outside the object.
    Any force from outside that surface is external.
    This called a "free body diagram".
  4. Feb 20, 2008 #3
    If you detemine the fingers as external forces, I think you can also look at the spring example as the solution of a system with two edge conditions determining the length of the spring. The Tension and the disformation of the spring hence form under x0=0 and x1=l, without regard to any external force.
  5. Feb 20, 2008 #4


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    What is "internal" and what "external" depend upon what the "system" is. You can always expand a "system" to include any outside forces.
  6. Feb 20, 2008 #5

    Andy Resnick

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    The "official" definition of an internal force is: the forces within in a material which act on an imaginary surface within the material. For your stretched elastic band, the internal forces are at equilibrium; cutting the elastic band will create unbalanced forces, causing the elastic band to move. Note that you can cut the band anywhere, and the band will essentially behave the same.

    Internal forces are also called 'contact' forces, IIRC.
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