Angular momentum | Wikipedia audio article | Wikipedia audio article
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Date: 18/11/2018

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Angular momentum | Wikipedia audio article

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SUMMARY
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In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity—the total angular momentum of a system remains constant unless acted on by an external torque.
In three dimensions, the angular momentum for a point particle is a pseudovector r × p, the cross product of the particle's position vector r (relative to some origin) and its momentum vector p = mv. This definition can be applied to each point in continua like solids or fluids, or physical fields. Unlike momentum, angular momentum does depend on where the origin is chosen, since the particle's position is measured from it. The angular momentum vector of a point particle is parallel and directly proportional to the angular velocity vector ω of the particle (how fast its angular position changes), where the constant of proportionality depends on both the mass of the particle and its distance from origin. For continuous rigid bodies, though,  the spin angular velocity ω is proportional but not always parallel to the spin angular momentum of the object, making the constant of proportionality I (called the moment of inertia) a second-rank tensor rather than a scalar.
Angular momentum is additive; the total angular momentum of a system is the (pseudo)vector sum of the angular momenta. For continua or fields one uses integration. The total angular momentum of any rigid body can be split into the sum of two main components: the angular momentum of the centre of mass (with a mass equal to the total mass) about the origin, plus the spin angular momentum of the object about the centre of mass.
Torque can be defined as the rate of change of angular momentum, analogous to force. The conservation of angular momentum helps explain many observed phenomena, for example the increase in rotational speed of a spinning figure skater as the skater's arms are contracted, the high rotational rates of neutron stars, the Coriolis effect, and precession of tops and gyroscopes. Applications include the gyrocompass, control moment gyroscope, inertial guidance systems, reaction wheels, flying discs or Frisbees, and Earth's rotation to name a few. In general, conservation does limit the possible motion of a system, but does not uniquely determine what the exact motion is.
In quantum mechanics, angular momentum is an operator with quantized eigenvalues. Angular momentum is subject to the Heisenberg uncertainty principle, meaning that at any time, only one component can be measured with definite precision; the other two cannot. Also, the "spin" of elementary particles does not correspond to literal spinning motion.