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 an object is placed on a surface, the magnitude of the normal force is equal to the net force pressing against the surface. On a flat surface, when forces other than gravity are in equilibrium, the normal force is equal to gravitational force in magnitude and opposite in direction.