In physics, the product of the mass of a body
and its linear velocity. Angular momentum (of
a body in rotational motion) is the product
of its moment of inertia and its angular
velocity. The momentum of a body does not
change unless it is acted on by an external
force. The law of conservation of momentum is
one of the fundamental concepts of classical
physics. It states that the total momentum of
all bodies in a closed system is constant and
unaffected by processes occurring within the
system. An orbiting body possesses angular
momentum, this being the result of the
combined effects of its orbital velocity,
mass, and distance from the primary. Angular
momentum is also a property of rotating
bodies. The angular momentum of an object of
mass m travelling at a velocity v in a
circular orbit of radius R is expressed as
mvR. Angular momentum is conserved, and
should any of the values alter (such as the
radius of orbit), the other values (such as
the velocity) will compensate to preserve the
value of angular momentum, and that lost by
one component is passed to another. An
example is a rotating gas cloud: any large
contracting gas cloud undergoes an increase
in its axial rotation velocity to conserve
angular momentum. Another example is a
spinning ice skater. With outstretched arms,
the skater spins only slowly; the rate of
spin increases dramatically when the arms are
pulled in, redistributing the mass of the
skater.