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.
Î´U = - W. In other words, work applied by a conservative force reduces the energy of configuration of a system (potential energy), converting it to kinetic energy. To see exactly how this conservation works, let's derive the expression for the potential energy of a system acted upon by gravity. Consider a ball of mass m dropped from a height h. The only force acting upon the ball is gravity, so we know the system is conservative, since we proved it last section.