Lagrangians and P-Conservation

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In summary, the conversation is about a problem involving a particle experiencing a central force and a gravitational force. The individual has already calculated the Lagrangian and equations of motion, and is now trying to determine if the system follows conservation of angular momentum. This can be checked by seeing if the system is invariant of rotation, and if \frac{\partial L}{\partial q} =0, then the conjugate momentum \frac{\partial L}{\partial \dot{q}} is a conserved quantity. If this is not clear, the individual is advised to post their Lagrangian for further clarification. It is also mentioned that the conjugate momentum is not always the same as the angular momentum. The individual is then instructed to compute
  • #1
moonman
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I have a problem with a particle experiencing a central force towards some origin, as well as a gravitational force downwards. I've calculated the Lagrangian, and the equations of motion. Now I'm being asked to see if the system follows conservation of angular momentum. How do I do this? I know it has something to do with seeing if the system is invariant of rotation, but how do I check for that?
 
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  • #2
If [tex]\frac{\partial L}{\partial q} =0[/tex] then the conjugate momentum [tex]\frac{\partial L}{\partial \dot{q}}[/tex] is a conserved quantity.

If that doesn't clear things up, then post what you have for the Lagrangian.
 
  • #3
But the conjugate momentum is the same as the angular momentum only in some cases.
Compute H and chech if H commutes with J.
 

1. What is a Lagrangian?

A Lagrangian is a mathematical function that describes the dynamics of a physical system in terms of its position, velocity, and time. It is used in the field of mechanics to determine the equations of motion for a system.

2. How is a Lagrangian different from a Hamiltonian?

A Lagrangian is based on the system's kinetic and potential energies, while a Hamiltonian is based on the system's total energy. The equations of motion derived from a Lagrangian are equivalent to those derived from a Hamiltonian, but the Lagrangian approach is often simpler and more elegant.

3. What is P-conservation?

P-conservation refers to the conservation of momentum in a physical system. This means that the total momentum of the system remains constant, even as individual objects within the system may change their momentum due to interactions with other objects.

4. How is P-conservation related to Lagrangians?

Lagrangians are used to derive the equations of motion for a physical system. These equations can then be used to determine whether momentum is conserved in the system. If the Lagrangian is invariant under translations in space and time, then momentum is conserved in the system.

5. Can Lagrangians be used to describe non-conservative systems?

Yes, Lagrangians can be used to describe both conservative and non-conservative systems. In non-conservative systems, energy may not be conserved, but the equations of motion can still be derived using the Lagrangian approach.

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