WOEK 177 



Before we can solve this problem, we must agree upon a defi- 

 nition of work and upon some means of measuring it. 



196. Work defined. It is always necessary to be sure that 

 we mean the same thing by the words we use ; therefore we 

 define work. We say that work is done when force is used to 

 move an object any distance ; as when a box is lifted from the 

 pavement into a wagon, or when a weight is pulled across the 

 floor. Pushing the lawn mower, sweeping, pumping water, 

 shoveling coal, are all forms of work, and in each case force 

 is used to make something move against resistance. 



We must guard ourselves against the error of thinking that 

 work is done when there is no motion ; as, for instance, when 

 the boy pulls on the end of the rope, but not forcibly enough 

 to move the boat, or when a person stands still and holds a 

 heavy object in his hand. In these cases it would be absurd 

 to speak of work being done, for in fact nothing is being ac- 

 complished, and at the end of a period of time everything is 

 just as it was at the beginning. In these cases there is effort 

 but no accomplishment, and therefore no work is done. 



197. Measurement of work. If a force of 1 pound is exerted 

 in moving an object through a distance of 1 foot, it is plain 

 that a certain small amount of work has been done. This 

 amount is called 1 foot-pound, and this is the unit of meas- 

 urement. If the distance is increased 2, 3, or 4 times, the 

 amount of work accomplished is increased proportionally. 

 Likewise, the increase in resistance and therefore in force 

 exerted is accompanied by a proportional increase in work. 

 Thus, if 1 foot-pound of work is done in lifting a pound weight 

 1 foot, it should be clear that 3 foot-pounds would be accom- 

 plished if the distance were 3 feet ; and if in the latter case the 

 weight were 2 pounds, the work done would be 2 times 3 foot- 

 pounds. That is, we calculate the work by multiplying the 

 force by the distance. 



198. Calculation of work in pulley systems. We may now 

 calculate work in and work out in the case of the pulley 



