COMMON TYPES OF WORK 181 



193. 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. 



194. 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 \foot-pound, and this is the unit of 

 measurement. If the distance is increased two, three, or four 

 times, the amount of work accomplished is increased propor- 

 tionally. 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 accomplished if the distance were 3 feet ; and if 

 in the latter case the weight were 2 pounds, the work done 

 would be 2 x 3 foot-pounds. That is, we calculate the work 

 by multiplying the distance by the force. 



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

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

 system used in moving the boat in section 188, in which case 

 five and not six strands of cord are used. If we suppose 

 that a pull of 500 pounds was necessary to move the boat 

 and that it was moved 50 feet, the work accomplished was 

 25,000 foot-pounds. The applied force acted through a dis- 

 tance of 250 feet, which is five times the distance the resist- 

 ance was moved. The force necessary, which was 100 pounds, 

 was one fifth the resistance (neglecting friction). 



It has been pointed out in a preceding section that there 

 can be no frictionless machine. We must therefore include 

 friction in our calculations. It is clear that the resistance 



