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Angular Momentum: A Conserved Quantity |
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Spin is a form of angular momentum intrinsic to all particles. But angular momentum is also due to the rotation of one object around another. When the earth rotates around the sun, it has angular momentum not only from its "intrinsic" rotation, but also from orbiting around the sun. |
The total angular momentum, due to spin and rotational motion, is a useful quantity to keep track of because it is always unchanged. In an accelerator, when particles smash together, other particles are created. But the total value of angular momentum is the same before, during, and after the collision. The amount of angular momentum that goes in must come out. This is conservation of angular momentum.
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Conservation of Angular Momentum |
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| Two particles are hurled at each other with incredible energy. | When the particles annihilate, two new particles are created. | At this point, one might note that
two particles with spin 1/2 became two particles with spin 0. Where did all the angular momentum go? Isn't it supposed to be conserved? |
Angular momentum is still conserved, it just changed forms. First it was in the form of spin and now it is in the form of rotational angular momentum. Just like the earth and the sun, the particles have angular momentum from moving around the collision point. Thus, angular momentum was conserved. Spin angular momentum came in and rotational angular momentum came out, but the total amount of angular momentum remained unchanged. |