Angular momentum:

The angular momentum of the rigid body is defined as the product of moment of inertia and angular velocity. The angular momentum is the measure of "quantity of motion". Angular momentum is connected with a particle in motion. The motion is not required to be rotational motion, but any motion. Importantly, it is measured with respect to the fixed point.   It is similar to linear momentum and is subject to fundamental constraints of the conservation of the angular momentum principle if there is no external torque on the object. It is a vector quantity. If a system consists of various particles, the total angular momentum about the origin can be obtained by adding all angular momentum of the constituent particles. Angular momentum can be computed by multiplying the square of displacement r, the mass of particle and angular velocity.It is derived from the expression for angular momentum of the particle .The angular momentum of the particle of mass m with respect to the chosen origin can be given by

L = mvr sin θ

or more formally by vector product

L = r x p

The direction can be given by the right hand rule which would give L the direction out of the diagram. For an orbit, the angular momentum is conserved, and this leads to one of the Kepler's laws. For the circular orbit, L becomes

L = mvr

The Angular momentum is perpendicular to plane formed by the pair of position and linear momentum vectors or by the pair of velocity and position vector, depending upon the formula used. Besides this, it is perpendicular to each of the operand vectors. However, vector relation by itself does not tell which side of plane formed by operands is direction of torque.