ENERGY. 77 



one and ten pounds respectively. Lift the first weight to 

 the top of the first step. How much work have you per- 

 formed ? Perhaps you will answer, one pound of work. 

 Now place the second weight beside the first. How much 

 work did you perform in so doing ? Perhaps you will say 

 ten times as much as before, or ten pounds. Now lift 

 each of them another step, and then another, until they 

 rest on the top of the tenth step. To lift the heavier 

 weight the second, third, and subsequent times involved 

 each as much work as to lift it the first foot, but you 

 would hardly say that you had lifted a hundred pounds. 

 Still it is sure that to place it on the tenth step required 

 just ten times as much work as it did to place it on the first 

 step, or just one hundred times as much work as it did to 

 place the one pound weight on the first step. Moreover, 

 it is evident that the two elements of weight and 

 height are necessarily to be considered in measuring 

 the work actually performed. 



153. Units of Work; the Foot-pound. It 



is often necessary to represent work numerically; hence 

 the necessity for a unit of measurement. The unit com- 

 monly in use, for the present, in England and this country 

 is the foot-pound. A foot-pound is the amount of work 

 required to raise one pound one foot high against 

 the force of gravity. The work required to raise one kilo- 

 gram one meter high against the same force is called a 

 kilogram-meter. 



(a.) To get a numerical estimate of work, we multiply the number 

 of weight units raised by the number of linear units in the vertical 

 height through which the body is raised. A weight of 25 pounds, 

 raised 3 feet, or one of 3 pounds raised 25 feet, represents 75 foot- 

 pounds. A weight of 15 Kg. raised 10 m., represents 150 kilogram- 

 meters. 



