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.
When work is done by external forces (nonconservative forces), the total mechanical energy of the object is altered. The work that is done can be positive work or negative work depending on whether the force doing the work is directed opposite the object's motion or in the same direction as the object's motion.