<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> 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 . </P> <P> 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 . </P>

When is the angular momentum of a system conserved