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 a system of two interacting objects, each exerts itself on the other. Consequence is that potential energy is defined for the system. The potential energy mgh (see below) that the object possesses in the earthâs gravitational field is actually the potential energy of the earth-object system. If the object falls towards the centre of the earth, an equal magnitude of force acting on the earth will induce motion in the earth also.