XIII. WORK AND MACHINES 



MEANING OF THE TERMS WORK AND ENERGY 



1. Hold a book out at arm's length for a few minutes. 

 What sensation is noticeable in your arm after a time? 

 Regardless of the fatigue and exhaustion experienced in 

 this exercise, it is only when motion occurs in bodies, 

 or in the particles composing them, that the term work 

 as used in Physics is applicable. There was work done 

 in changing the position of the book but not in holding it 

 in place. 



2. Why do the muscles of the body become tired (fatigued) 

 through exercise when at labor or play ? Do machines such 

 as the steam engine become weary when at work? What 

 does, result in them from continued use ? 



3. Name several changes in bodies caused by the action 

 of forces upon them. In which of these cases is work done, 

 and why so ? 



4. What is the basis for a belief that there is any such 

 force as gravity? How does an intending purchaser of 

 a horse assure himself that the animal has sight (good 

 vision) ? What is the meaning of the term theory as used 

 in science, and in the affairs of life? What must be true 

 of any belief, opinion, or theory if we are justified in con- 

 tinuing to cherish it ? When must a theory be modified or 

 rejected ? 



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