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.
Forces that are equal in size but opposite in direction are called balanced forces. Balanced forces do not cause a change in the motion of objects. Forces that cause a change in motion of objects are called unbalanced forces. Unbalanced forces can change the motion of an object in two ways.