WORK AND MACHINES 



161 



MEASUREMENT OP WORK AND ENERGY 



1. With a spring balance lift some small body; note its 

 weight in pounds (and sixteenths of a pound). This value 

 represents not only the quantity of matter in the body, i.e., 

 its mass (ra), but 



as well the value 

 of gravity as a 

 force (/) pulling 

 down on the 

 mass, and the re- 

 sistance (r) over- 

 come in lifting 

 the body. In the 

 measurement of 

 work, as well as 

 of the energy 

 used in doing 

 that work, one of 

 the units em- 

 ployed is the foot- 

 pound. This is often defined as the amount of work done 

 (and of energy used in doing it) when a force of one pound 

 causes motion through one foot distance. State a correspond- 

 ing form of definition for work when the term resistance is 

 substituted for "force," and the phrase is overcome takes 

 the place of "causes motion." 



2. Calculate the number of foot-pounds done when the 

 reading of the balance is l^ 5 y pounds, and the distance the 

 body is lifted is 16 inches (l feet). 



FIG. 72. Work is the " overcoming of resistance." 



