CH. ix WORK AND ENERGY 119 



will convince the student that all instances of mechanical con- 

 trivances, by means of which work is accomplished, come within 

 the scope of our definition, as well as all other cases when we 

 say in ordinary language that work is done. Take, for instance, 

 a horse drawing a heavy weight along a road. Here the force 

 exerted by the horse is used up in overcoming the resistance 

 due to the road. A man raising a mass from the ground over- 

 comes the resistance due to the body's weight. A body falling 

 from a height under the influence of the earth's attractive force 

 has work done upon it, with the result that its velocity increases 

 according to a uniform acceleration of 32 '2 feet per second in 

 every second. 



Measurement of Work. Referring to our definition of work 

 it would seem as though we had two kinds of work to measure, 

 viz. , the ivork of acceleration and the work against resistance. But 

 since we can make a force perform either of these kinds of work 

 according to the condition under which it acts, it is possible to 

 measure either of them in the same units. An example will 

 make this clearer. We can either allow a mass to drop from 

 the hand and to move freely through the air with the uniform 

 acceleration we have mentioned above until it reaches the ground ; 

 or we can attach the mass to a string, pass the string over a 

 cylinder, and allow it to move towards the earth with a small 

 uniform velocity a result which can be brought about by 

 applying the necessary friction between the cord and the cylinder, 

 that is by applying a resistance. The final result brought about 

 is the same under both sets of conditions ; but in the first case 

 the work is of acceleration, while in the second it is work against 

 resistance. 



Unless there is motion no work is done. If we put a weight 

 upon a table or shelf, so long as the weight remains in one place, 

 it evidently does no work, though it is capable of doing work 

 by reason of its elevated position. 



For practical purposes the unit of work which is adopted is 

 the work done in raising the mass of one pound through one 

 foot, and it is called the foot-pound. This is not a strictly 

 constant unit, for it will be evident, in the light of what has 

 been said about the weight of a body, that where the weight 

 is greater the amount of work done will be greater. The 

 unit of work will vary slightly in different latitudes in a 

 precisely similar manner to that in which the weight of a mass 

 varies. 



