CHAPTER VII 

 WORK 



109. Measurement of work. There are various kinds of work, 

 but in mechanics we are concerned only with the work done in 

 moving bodies which are acted on by forces. Such work is 

 described as mechanical work. We say that mechanical work is 

 done whenever a body is moved in opposition to the forces acting 

 on it, as, for instance, in raising a weight, in dragging a heavy 

 body over a rough surface, or . in stretching an elastic string. In 

 the first case work is performed against the force of gravity, in 

 the second case against the frictional force exerted on the moving 

 body by the rough surface, and in the third case against the 

 tension of the string. 



Obviously in estimating the amount of work done, two factors 

 have to be taken into account, namely the amount of the force 

 acting on the body and the distance through which the body is 

 moved in opposition to this force. The amount of work will clearly 

 be directly proportional to the force, in raising a weight of 200 

 pounds through a given distance we do twice as much work as 

 in raising a weight of 100 pounds through the same distance. It 

 will also be proportional to the distance moved, hi raising a 

 weight through two feet we do twice as much work as in raising 

 the same weight through one foot. Thus the amount of work done 

 varies as the product of the force and the distance. 



The amount of work done in raising a weight of one pound 

 through a height of one foot is called one foot pound. 



From what has been said, it is clear that the work done in 

 raising a weight of w pounds through a height of h feet is 

 wh foot pounds. Also, the work done in moving a body a distance 

 of s feet in opposition to a force of F pounds weight is Fs foot 



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