MACHINES 



157 



The amount of work done depends upon the force used 

 and the distance covered (sometimes called displacement), and 

 hence we can say that 



or 



Work = force multiplied by distance, 

 J>F = x d. 



151. Machines. A glance into 

 our machine shops, our factories, 

 and even our homes shows how 

 widespread is the use of complex 

 machinery. But all machines, how- 

 ever complicated in appearance, are 

 in reality but modifications and com- 

 binations of one or more of four 

 simple machines devised long ago 

 by our remote ancestors. These 

 simple devices are known to-day, as 

 (i) the lever, represented by a crow- 

 bar, a pitchfork ; (2) the inclined 

 plane, represented by the plank 

 upon which barrels are rolled into a 

 wagon ; (3) the pulley, represented 

 by almost any contrivance for the 

 raising of furniture to upper stories ; 

 (4) the wheel and axle, represented 

 by cogwheels and coffee grinders. 



Suppose a 6oo-pound bowlder which is embedded in the 

 ground is needed for the tower of a building. The problem 

 of the builder is to get the heavy bowlder out of the ground, 

 to load it on a wagon for transportation, and finally to raise 

 it to the tower. Obviously, he cannot do this alone ; the 

 greatest amount of force of which he is capable would not 

 suffice to accomplish any one of these tasks. How then does 



FlG. 95. Primitive method of 

 grinding corn. 



