Angular momentum

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The angular momentum of a point mass about a point is defined as where r is the position vector of the point mass with respect to the point of reference and p is the linear momentum vector of the point mass.

The principle of angular momentum can be applied to a system of particles by summing the angular momentum of each particle about the same point.

The derivative of angular momentum with respect to time is equal to the sum of the external moments (or torque ) applied to the system. Differentiating angular momentum gives:

For a constant radius, the second term is zero. Hence From this, it can be concluded that in the absence of an external moment, angular momentum must be conserved.