What Are Internal and External Forces in Materials?

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Discussion Overview

The discussion revolves around the concepts of internal and external forces in materials, with a focus on defining these terms and understanding their implications through examples, particularly using an elastic band. The scope includes conceptual clarification and technical explanation.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes an example involving an elastic band to illustrate the difference between external forces (the pull from fingers) and internal forces (the tension within the elastic).
  • Another participant introduces the concept of a "free body diagram" to define external forces as those acting on a surface just outside an object.
  • A different viewpoint suggests that the spring example can be analyzed as a system with edge conditions, where tension and deformation occur independently of external forces.
  • It is noted that the definitions of internal and external forces depend on the chosen "system," implying that any outside forces can be included by expanding the system's boundaries.
  • One participant reiterates their initial example and provides an "official" definition of internal forces, emphasizing their role within a material and their equilibrium state.
  • Internal forces are also referred to as 'contact' forces by one participant, though this is presented with some uncertainty.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of internal and external forces, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some definitions and examples provided may depend on specific assumptions about the system being analyzed, and there are unresolved nuances regarding the characterization of internal forces.

DaveC426913
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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?
 
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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".
 
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.
 
What is "internal" and what "external" depend upon what the "system" is. You can always expand a "system" to include any outside forces.
 
DaveC426913 said:
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?

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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