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.
The magnitude of the net force acting on an object is equal to the mass of the object multiplied by the acceleration of the object as shown in the formula below. If the net force acting on an object is zero, then the object is not accelerating and is in a state that we call equilibrium. When an object is in equilibrium, then two things can be true: either the object is not moving at all, or the object is moving with a constant speed.