Angular Momentum

Objects executing motion around a point possess a quantity called angular momentum. This is an important physical quantity because all experimental evidence indicates that angular momentum is rigorously conserved in our Universe: it can be transferred, but it cannot be created or destroyed. In accreting binaries, conservation of angular momentum often causes the formation of an accretion ring.

For the simple case of a small mass executing uniform circular motion around a much larger mass, the amount of angular momentum takes a simple form. As the adjacent figure illustrates, the magnitude of the angular momentum in this case is L = mvr, where L is the angular momentum, m is the mass of the small object, v is the magnitude of its velocity, and r is the separation between the objects.

Because the above formula can be rearranged to give v = L/(mr) and L is a constant for an isolated system, the velocity v and the separation r are inversely correlated. Thus a decrease in the separation r is accompanied by an increase in the velocity v, and vice versa.