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angular momentum
The momentum that an object or system of objects has because of its rotation. It is a vector quantity directed along the axis of rotation.
Since angular momentum is a conserved quantity in physics, the angular momentum of an orbiting body must stay the same at all points in the orbit. Orbital angular momentum is given by multiplying together a body’s mass (m), its orbital angular velocity (v), and the distance (r) from the body around which it is moving. Since m is constant, v increases as r decreases (and vice versa) in an elliptical orbit. This is why planets in the Solar System, for example, travel faster at perihelion than they do at aphelion. Angular momentum is also conserved for an object spinning on its own axis (like a pirouetting ice skater), which explains why stars spin more slowly as they expand and faster as they contract (although any mass loss will carry away some angular momentum).
In quantum mechanics, angular momentum is quantized in units of Planck's constant (divided by 2π). This corresponds classically to only certain frequencies of rotation being allowed. One implication of this is that if electrons are thought of as being in orbit around the central nucleus of an atom. Similarly, the spin of an electron is quantized, and the electron can only exist with spin up (+½) or spin down (-½) measured against some external reference such as a magnetic field.
Related categories
• CLASSICAL MECHANICS
• PARTICLE PHYSICS
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