Conservation of angular momentum

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Discussion Overview

The discussion centers on the conservation of angular momentum within a system, particularly in scenarios involving multiple bodies and the effects of torque. Participants explore the implications of net torque on the conservation of angular momentum for both the system as a whole and its individual components.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that if there is no net torque acting on a system, the total angular momentum of the system is conserved, while others question the conservation of angular momentum for individual bodies within the system.
  • One participant proposes a scenario where one body produces torque and the other does not, suggesting that the angular momentum of the non-torque-producing body is conserved, which is challenged by others who argue this situation is not possible.
  • Another participant emphasizes that the angular momentum of a closed system is constant, but this does not imply that the angular momentum of each component is independently conserved, as interactions can lead to the transfer of angular momentum between components.
  • There is a discussion about the interpretation of "producing torque," with some participants finding it difficult to understand how one body can exert torque without affecting the other.
  • One participant highlights that the conservation of angular momentum applies to the system as a whole rather than to individual bodies, suggesting that the conservation laws are contingent on the interactions within the system.
  • Another participant draws an analogy to conservation of energy, indicating that while individual energies may change, the total remains constant, similar to angular momentum in the presence of internal torques.

Areas of Agreement / Disagreement

Participants express disagreement regarding the conservation of angular momentum for individual bodies versus the system as a whole. There is no consensus on the interpretation of torque and its implications for angular momentum conservation in the discussed scenarios.

Contextual Notes

Participants note that the definitions of the system and the nature of the forces and torques involved are crucial to understanding the conservation principles being discussed. The conversation reveals complexities in the interactions that may affect angular momentum conservation.

gracy
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If there is no net torque acting on a system total angular momentum of the system will be conserved as well as angular momentum of each body present in the system will be conserved.
And if there are two bodies /two charges present as a system and one of them (let's say body 1 )produces torque about a point and the other (body2)does not ,then in this case angular momentum of body 2 is conserved and angular momentum of body 1 isn't conserved.Right?
 
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gracy said:
as well as angular momentum of each body present in the system will be conserved.
Why would you say that? How are you defining your 'system' and what is the torque acting on?
 
gracy said:
And if there are two bodies /two charges present as a system and one of them (let's say body 1 )produces torque about a point and the other (body2)does not ,then in this case angular momentum of body 2 is conserved and angular momentum of body 1 isn't conserved.Right?
This is not a possible situation.

Edit: One assumes that you are using the scenario from your other thread where the two bodies in question are interacting and the relevant force is a third law pair between the two.
 
The angular momentum of a closed system is constant. This does not mean that the angular momentum of each component of the system is independently conserved. Angular momentum can be traded around between elements of the system when interactions occur where torques arise.

The analogous situation holds for linear momentum. When objects collide elastically the momentum of the individual objects is not conserved (they can have different velocities before and after the collision) but the total momentum is conserved.
 
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jbriggs444 said:
This is not a possible situation.
What do you mean ?You mean it is wrong .
 
gneill said:
the angular momentum of each component of the system is independently conserved.
This never happens.I mean conservation of momentum /angular momentum is always for a system not for individual bodies?
 
gracy said:
What do you mean ?You mean it is wrong .
A third law force pair cannot exert a non-zero torque on the one endpoint without exerting an equal and opposite non-zero torque on the opposite endpoint.
 
gracy said:
What do you mean ?You mean it is wrong .
I was asking about this
gracy said:
And if there are two bodies /two charges present as a system and one of them (let's say body 1 )produces torque about a point and the other (body2)does not ,then in this case angular momentum of body 2 is conserved and angular momentum of body 1 isn't conserved.Right?
 
gracy said:
I was asking about this
The most plausible interpretation of that quoted passage was that you were talking about two interacting bodies where the force of one on the other produced a torque but the force of the other on the one did not.

The notion of a body "producing a torque" is difficult to interpret. You may have had something else in mind.
 
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gracy said:
gneill said:
that the angular momentum of each component of the system is independently conserved.
This never happens.I mean conservation of momentum /angular momentum is always for a system not for individual bodies?
@gracy
You really need to do a better job of quoting. From the above it looks as if gneill is agreeing with the first sentence of the OP, when in fact the complete sentence this quote came from said: (Emphasis is mine.)
gneill said:
This does not mean that the angular momentum of each component of the system is independently conserved.
That follows the sentence,
The angular momentum of a closed system is constant.​

I don't mean to imply that you intended to mislead. It's just that it can be very difficult for some of us to figure out exactly what it is that you are asking in your questions.

Now, to answer your question: The issue of whether or not the angular momentum of individual components (perhaps bodies) of the system are conserved depends upon details of the internal interactions of the system. Under some circumstances, the angular momentum of the systems components might be conserved.

There can be a lot of room between always and never.
 
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  • #11
The key word is net. If there is no net torque acting on the system, its angular momentum remains constant. So there might be a torque on something in the system but if there is an equal and opposite torque on the same body or another one, the angular momentum of the system of bodies is the same.

It's like the conservation of energy. If I throw a ball into the air, the total energy it has might not stay the same because it loses some to the air in the form of heat. But, the energy of the air around it will increase because the particles in the air gain more kinetic energy. So the sum of the energy of the ball and the air will stay constant even though their individual energies may not. It's the same with the conservation of angular momentum.
 

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