Commutation of Angular and Linear Momentum

In summary, the conversation discusses rewriting a relation as an identity and evaluating it using the ABC rule. It is suggested to explicitly write out the dot product and apply the correct version of the ABC rule.
  • #1
atomicpedals
209
7
If I have a relation such as [tex][L_{j} , \vec{p}^2]=0[/tex] where j=x,y,z.

Can I re-write it as [tex][L_{j}, \vec{p} \vec{p}]=0[/tex] and then evaluate it as though it were an identity? e.g. [tex][A,BC]=[A,B]C+[B,A]C=...[/tex]
 
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  • #2
atomicpedals said:
If I have a relation such as [tex][L_{j} , \vec{p}^2]=0[/tex] where j=x,y,z.

Can I re-write it as [tex][L_{j}, \vec{p} \vec{p}]=0[/tex] and then evaluate it as though it were an identity? e.g. [tex][A,BC]=[A,B]C+[B,A]C=...[/tex]

If unsure, you are better off explicitly writing out the dot product:
[itex][ L_j, \vec{p}^2] = [L_j, \sum_{k} p_k^2] = \sum_{k} [L_j, p_k^2][/itex]

Now you can apply the ABC rule. The one you quote is wrong, by the way. It should be:
[itex][A, BC] = B[A,C] + [A,B]C[/itex]
 
  • #3
Thanks for the help!
 

FAQ: Commutation of Angular and Linear Momentum

What is the principle of conservation of angular and linear momentum?

The principle of conservation of angular and linear momentum states that the total angular and linear momentum of a system remains constant in the absence of external forces and torques. This means that the total amount of angular and linear momentum in a system cannot be created or destroyed, but can only be transferred between objects within the system.

How is angular momentum defined?

Angular momentum is a measure of the rotational motion of an object. It is defined as the product of an object's moment of inertia and its angular velocity. Mathematically, it can be represented as L = Iω, where L is the angular momentum, I is the moment of inertia, and ω is the angular velocity.

What is the relationship between angular and linear momentum?

Angular and linear momentum are related by the commutation of angular and linear momentum. This means that the change in one type of momentum is equal to the cross product of the other type of momentum and the position vector between the two objects. Mathematically, it can be represented as dL/dt = r x F, where dL/dt is the change in angular momentum, r is the position vector, and F is the force applied.

How does the conservation of angular and linear momentum apply to real-world scenarios?

The conservation of angular and linear momentum is a fundamental law of physics and applies to all real-world scenarios. For example, when a spinning ice skater pulls her arms in, her angular velocity increases, but her moment of inertia decreases, resulting in a constant total angular momentum. Similarly, in a collision between two objects, the total angular and linear momentum before and after the collision must be equal.

Can the commutation of angular and linear momentum be applied to non-rigid bodies?

Yes, the commutation of angular and linear momentum can be applied to non-rigid bodies as well. In this case, the moment of inertia and angular velocity may change as the object deforms, but the total angular and linear momentum will still remain constant. This principle is often used in the study of rotating systems, such as planets and galaxies.

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