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 physics, a force is an external cause responsible for any change of a physical system. For instance, a person holding a dog by a rope is experiencing the force applied by the rope on his hand, and the cause for its pulling forward is the force exercised by the rope.