A rotating system of two point particles with inner torque

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SUMMARY

The discussion centers on the concept of inner torques in a system of two point particles rotating around an axis. The torques are defined as M21 = r1 x F21 and M12 = r2 x F12, but the participants question the necessity of considering inner torques, given that the forces should cancel each other out according to Newton's Third Law. The confusion arises from the lack of clarity regarding the attraction between the particles and the relevance of inner torques in deriving a general equation for rotating systems. Ultimately, the participants seek a clearer understanding of the relationship between forces and torques in this context.

PREREQUISITES
  • Understanding of Newton's Third Law of Motion
  • Familiarity with torque calculations in physics
  • Basic knowledge of point particle systems
  • Concept of rigid body dynamics
NEXT STEPS
  • Study the derivation of torque equations in rotating systems
  • Explore the implications of Newton's Third Law in multi-body systems
  • Investigate the relationship between forces and torques in rigid body dynamics
  • Learn about the mathematical modeling of point particles in physics
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Physics students, educators, and researchers interested in the dynamics of rotating systems and the application of torque in multi-body interactions.

bolzano95
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Lets say we have a system of two point particles (1. and 2.) which are rotating around an axis.
IMG_8329.jpg
What is written next in my physics course book is: The torque of a 2.body on the 1. body is M21=r1xF21 and the torque of the 1.body on the 2.body is M12=r2xF12. Understandable.
But how? There is no mention or suggestion that those two particles are attracted each other.

I don't understand why are we having inner torques in this example- we are dealing with a system: and it's normal that the inner forces cancel each other out based on the 3rd Newtons Law.
So why are we dealing with inner torques?

I think it's useless thinking about inner torques in the system: if we divide a rigid body to tiny pieces and we take them as point particles, the sum of inner torques is 0.

But the author is trying to derive a general equation for a rotating system of point particles and a rigid body. But he starts explaining the theory in such a confusing way that I'm not sure what I'm missing.
 
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bolzano95 said:
But how? There is no mention or suggestion that those two particles are attracted each other.
Aren't the forces shown in the diagram enough of a suggestion?
 

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