Public School, Deer Creek Minnesota, 1918 LakesnWoods.com Postcard and Postcard Image Collection - click image to enlarge Aerial view, Deer Creek Minnesota, 1972
Whenever work is done upon an object by an external force, there will be a change in the total mechanical energy of the object. If only internal forces are doing work (no work done by external forces), there is no change in total mechanical energy; the total mechanical energy is said to be conserved.
In this case, must be zero, implying that the total angular momentum of a system is constant. We can state this verbally: If no net external torque acts on a system, the total angular momentum of the system remains constant. This statement describes the conservation of angular momentum. It is the third of the major conservation laws encountered in mechanics (along with the conservation of energy and of linear momentum). There is one major difference between the conservation of linear momentum and conservation of angular momentum. In a system of particles, the total mass cannot change. However, the total moment of inertia can. If a set of particles decreases its radius of rotation, it also decreases its moment of inertia.