MECHANICAL PRINCIPLES OF MACHINES 25 



screw. The force exerted on the mechanical principle is 

 called acting force or power, and that given out is called 

 weight or resisting force. We must bear in mind that none 

 of these simple machines or mechanical elements can gen- 

 erate energy, but that they enable energy to be distributed 

 and utilized to the best advantage. As an illustration, the 

 ability to work hard and without rest varies according to 

 the manner in which a workman applies his force, and the 

 number of muscles he brings into action. In the operation 

 of turning a crank, a man's strength changes in every part 

 of the circle which the handle describes. It is greatest when 

 he pulls the handle upward from the height of his knees, 

 and weakest at the top and bottom of the circle, where the 

 handle is pushed or drawn horizontally. 



Questions 



1. Is it possible to determine the degree of skill of a trade by 

 the number of tools used? Explain. 



2. What impression would you gather from a person who was 

 driving carpet tacks with a machinist's hammer? Explain. 



3. Is it economical to use a sledge hammer to drive ordinary 

 wire nails into a board floor? Explain. 



4. State the kind of energy used in the following cases: (a) a 

 man lifts a casting from the floor; (b) a house is moved with horses; 

 (c) a grist mill grinds corn by means of running water back of the 

 mill; (d) a steam boiler drives the engine. 



5. Has a mechanic more energy in the morning before going to 

 work than after a day's work? Explain. 



