SIMPLE MACHINES 177 



condition or position of an object is called potential 

 energy. Energy of an object which is due to motion is 

 called kinetic energy. Energy is ability or capacity to 

 move an object. To walk requires energy because our 

 body is the object moved. It requires energy to move 

 a wagon, a car, or a train. Energy, like heat, cannot be 

 thought of apart from some object or substance. 



(B) Force. We never use at one time all of the 

 energy stored in our body, but only a part of it at one 

 time. When we walk, throw a ball, turn a machine, or 

 lift a book we use part of the energy of our body. 

 The part of the energy that we use at any one 

 time is called force. So force is the amount of effort 

 exerted at any one time. Or, force is the energy which 

 is in process of use ; or, force is the energy which is being 

 used to move or hold an object. Force, then, like energy, 

 cannot be thought of apart from matter or objects. 



To hold in the hand an iron ball which weighs a pound 

 requires us to exert a "pound of force." A pound of 

 force is the force exerted by the earth on a pound of mass 

 or matter in pulling the matter toward its center. The 

 earth's force or "pull" on an object is called gravity. 

 To hold in the hand a ball which weighs a gram requires 

 one to exert a "gram of force." The pound of force and 

 the gram of force are two units of force. There is no 

 unit of energy because we do not measure energy except 

 when it appears in the for,m of force or heat, and then 

 we call it force or heat respectively. 



(C) Work. If a 2oo-pound force is exerted upon a 

 barrel weighing 400 pounds, no work is done if the barrel 

 does not move. Holding a book in one place is not 

 working. In these instances force is used, but no work 

 is done. Picking up a book from the floor and placing 



