How a rigid body causes a reaction force?

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Discussion Overview

The discussion revolves around the nature of reaction forces in rigid bodies, particularly in the context of Newton's third law of motion. Participants explore whether reaction forces arise from molecular interactions, deformation, or conservation laws, and whether these concepts can be reconciled without invoking molecular mechanics.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants suggest that reaction forces are fundamentally linked to the deformation of bodies when forces are applied.
  • Others argue that reaction forces can be understood through Coulombic forces at the molecular level, even in rigid bodies.
  • A participant mentions that momentum conservation is a key principle that underlies Newton's third law, suggesting that the law is independent of specific forces like gravity or Coulomb forces.
  • There is a discussion about whether Newton's third law can be derived from Newton's second law, with some expressing skepticism about this equivalence.
  • One participant highlights that in practical scenarios, such as pushing an object in zero gravity, the action-reaction principle is more observable compared to situations on Earth where friction complicates the observation.
  • Another participant notes that while rigid bodies are often modeled as such, true rigidity is not achievable, as all bodies deform to some extent under force.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the origin of reaction forces, with no consensus reached on whether these forces arise from deformation, molecular interactions, or conservation laws.

Contextual Notes

Some discussions reference specific physical scenarios and theoretical concepts, such as Noether's Theorem and conservation laws, which may not be universally understood by all participants. The implications of friction and the practical observation of forces in different environments are also noted as factors complicating the discussion.

  • #61
I'm diagreeing with the idea, not to answer a question according to the known facts. Of course, when learning about classical mechanics in the very first semesters you cannot explain relativity and quantum mechanics in all detail, but you can tell the students already then, in a qualitative way, as I tried in my answers above, that you need more advanced physics to answer the question. After all, we teach classical mechanics not so much for its own sake but as the preparation for the more advanced and up-to-date topics of modern physics.

For me the main justification to teach the fascinating subject of rigid bodies and spinning tops is to introduce the rotation group as a Lie group and use Lie-algebra arguments to derive the equations of motion using Hamilton's principle (at my university it's usualy taught in the 2nd semester in the 2nd theory-course lecture, where analiytical mechanics is treated). It's a great opportunity to introduce these quite advanced topics at the example of a non-trivial but fascinating phenomenon.
 
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  • #62
vanhees71 said:
(at my university it's usualy taught in the 2nd semester in the 2nd theory-course lecture,
I have to agree that the Thread Header is I so it is probably appropriate. However, the question about an infinitely strong rope doesn't fit in with that and I was suggesting that the point couldn't be appropriately answered by just digging deeper.
 

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