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 product of average force and the time it is exerted is called the impulse of force. From Newton's second law. the impulse of force can be extracted and found to be equal to the change in momentum of an object provided the mass is constant:f an impact stops a moving object, then the change in momentum is a fixed quantity, and extending the time of the collision will decrease the impact force by the same factor. This principle is applied in many common-sense situations: