CHAPTER VII. 



FORCE AND ENERGY. 



FORCE is that which causes motion or changes motion. While 

 this definition conveys no idea of what force is, it does give us 

 some idea of what force does. Force may be measured by an in- 

 strument called a dynamometer, of which the common draw 

 scale is an illustration. When a body is moved by the action of 

 force upon it, the force is said to do work on the body. For ex- 

 ample, steam exerts pressure on the piston of a cylinder of an 

 engine, causing it to move. The expansive force of the steam 

 does work on the piston in overcoming resistance and putting it 

 in motion. If there had been no motion of the piston no work 

 would have been done. In merely supporting a body no work is 

 done, since no motion is produced. The force of gravity does no 

 work on the stone resting on the ground, yet it causes a pressure 

 between the stone and the earth. Force and space are essential 

 elements of work. A force acting through a space of one foot in 

 raising a weight does a certain amount of work. The same force 

 acting through a space of two feet would do twice as much work. 

 That is, in general, force multiplied by the space through which 

 it acts equals the work done. Formula : Force, F. X Space, S. 

 work, W. 



Every moving body can impart motion; can do work on any 

 other body, and is said to possess energy. The energy of a body 

 is its capacity to do work. For example, the moving bat in the 

 hands of a ball player imparts motion to the ball ; it does work 

 on the ball. The energy which a body, as the bat, possesses, 

 in consequence of its motion, is called kinetic energy. A stone 

 lying on the ground is devoid of energy. Do work upon the 

 stone by lifting it up to a shelf, or other support, and it seems as 

 devoid of energy as when lying on the earth. Attach one end of 

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