540 MACHINES, WORK, AND ENERGY 



toward the center of the earth, forces may act in any direction 

 whatever. A team of horses pulling a plow exerts a force, 

 but the direction of the force is in a nearly horizontal line. A 

 locomotive usually exerts a pull, i.e., a force, which is almost 

 exactly horizontal. A boy may throw a ball in any direction 

 he chooses, but in doing so he exerts a force upon it. Weight, 

 then, is a term which we apply to a force due to a certain cause 

 the pull of the earth and is always acting in a vertical line. 

 Before we go further in this study of machinery we should be 

 certain that we clearly understand the difference between: 



A pound-mass, a pound-weight, and a pound-force; 



A gram-mass, a gram- weight, and a gram-force; 



A kilogram-mass, a kilogram-weight, and a kilogram-force. 



636. Work; By WORK we mean a push or a pull acting 

 through distance. The table or desk which supports your 

 books does no work. The columns which support the porch 

 roof do no work. A man attempting to lift a piano which he 

 is unable to lift, or a team of horses attempting to pull a loaded 

 wagon which it is unable to move, does no work. As the 

 term work is used in mechanics, no person or machine does work 

 unless a force actually acts through space. A force exerted 

 in an attempt to move an object which does not move is 

 WASTED EFFORT but no work is done. 



This use of the term work is not peculiar to mechanics; it is 

 really the common, everyday meaning of the term. When a 

 man lets the contract for the building of a house, he agrees to 

 pay for the work done, never for effort put forth. The contrac- 

 tor, in turn, pays his men for the work they actually do. Even 

 though the employer pays his men by the day, he continues to 

 employ only those who actually accomplish the required amount 

 of work. 



637. The Units of Work. The common English unit of work 

 is the FOOT-POUND. The FOOT-POUND is the amount of work 

 done by a force of one pound acting through a distance of one 

 foot. Since a 1-lb. mass weighs 1 lb. ; we do 1 ft.-lb. of work 



