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.
1 The forces you hit feather and feather hits you are a Newton's Third Law action/reaction force pair, so they must always be exactly equal in size. 2  If an object isn't strong enough to exert a certain amount of force on you, then you can't exert that amount of force on it. Call this the action force, and notice that this force pushes the Earth-not you! 2  According to Newton's Third Law, the reaction force in this interaction must be Earth pushes you upward, and this is the force that causes you to accelerate into the air.