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.
It is thus possible for forces to act on an object, yet have the net force be zero. When evaluating the motion caused by forces acting on an object, remember to find the vector sum of those forces. Also included in Newton's First Law, though not explicitly, is the concept of inertia. Inertia is defined as the tendency of an object to remain at a constant velocity.