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Conservation of angular momentum

  1. Jan 21, 2016 #1
    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?
  2. jcsd
  3. Jan 21, 2016 #2


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    Why would you say that? How are you defining your 'system' and what is the torque acting on?
  4. Jan 21, 2016 #3


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    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.
  5. Jan 21, 2016 #4


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    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.
  6. Jan 21, 2016 #5
    What do you mean ?You mean it is wrong .
  7. Jan 21, 2016 #6
    This never happens.I mean conservation of momentum /angular momentum is always for a system not for individual bodies?
  8. Jan 21, 2016 #7


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    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.
  9. Jan 21, 2016 #8
    I was asking about this
  10. Jan 21, 2016 #9


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    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.
  11. Jan 22, 2016 #10


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    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.)
    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.
  12. Jan 23, 2016 #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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