Angular Momentum: Cycle Wheel & Rod System

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Discussion Overview

The discussion revolves around the angular momentum and moment of inertia of a system consisting of a cycle wheel with attached rods and masses. Participants explore the relationships between the angular momentum of the whole system and its components, as well as the moment of inertia for the system and its parts.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asks whether the angular momentum of the whole system can be considered as the sum of the angular momentum of the wheel and the angular momentum of the two masses.
  • Another participant agrees, stating that angular momentum is a vector quantity and can be summed accordingly.
  • A question is raised about the applicability of the same principle to moment of inertia, specifically whether the moment of inertia of the wheel plus the moment of inertia of the masses equals the moment of inertia of the whole system.
  • One participant confirms that the moment of inertia of a system is the sum of the moments of inertia of its parts, but notes that the moment of inertia of the cross piece should also be considered if it is not negligible.
  • Another participant emphasizes the importance of ensuring that the centroids of the composite shapes are along the same axis when summing moments of inertia, mentioning the parallel axis theorem as a necessary consideration.
  • A later reply rephrases the statement about moment of inertia, clarifying that it applies about the same axis.

Areas of Agreement / Disagreement

Participants generally agree on the principles regarding angular momentum and moment of inertia, but there are nuances regarding the conditions under which these principles apply, particularly concerning the cross piece and the alignment of centroids.

Contextual Notes

Participants highlight the need to consider the parallel axis theorem and the centroids' alignment when discussing the moment of inertia, indicating that assumptions about the system's configuration may affect the validity of the claims.

rgshankar76
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I have a system that consists of a cycle wheel on which a vertical thin rod is welded. to this rod another thin rod is welded perpendicular. It is something like a cross fitted on the wheel and as the wheel rotates, whole thing rotates. now on the top rod if two eqaul masses are slided one from each side and fixed. Can i take the angular momentum of the whole system to be angular momentum of the wheel + the angular momentum of the two mass system?
 
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rgshankar76 said:
I have a system that consists of a cycle wheel on which a vertical thin rod is welded. to this rod another thin rod is welded perpendicular. It is something like a cross fitted on the wheel and as the wheel rotates, whole thing rotates. now on the top rod if two eqaul masses are slided one from each side and fixed. Can i take the angular momentum of the whole system to be angular momentum of the wheel + the angular momentum of the two mass system?
Yes. Angular momentum is a vector quantity. The angular momentum vector of the whole is equal to the sum of the angular momenta vectors of its parts.

AM
 
moment of inertia

is it also applicable in the case of moment of inertia? means, MI of wheel + MI of masses = MI of whole system?
 
Yes. The MOI of a system = Sum of the MOIs of its parts. (You seem to be ignoring the MOI of the cross piece. If it's light enough, that may be OK.)
 
Yes. The MOI of a system = Sum of the MOIs of its parts. (You seem to be ignoring the MOI of the cross piece. If it's light enough, that may be OK.)

Be carefull! This is true, IF the centroids of the composite shape are along the same axis! In general, you can add them and subtract them, but you have to include the parallel axis theorem as well!
 
I'll rephrase my statement: The MOI of a system about an axis = Sum of the MOIs of its parts about that same axis. :smile:
 

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