180 



SIMPLE MACHINES 



the power must be exerted 

 through one complete turn of 

 the crank. A derrick (Fig. 180) 

 consists of a winch and a 

 system of cog wheels and 

 pulleys. 



203. The Inclined 

 Plane. In climbing a 

 hill or mountain we pre- 

 fer to go up gradual 

 slopes instead of steep 

 ones, because in this way 

 we can raise the weight of our bodies with much less 

 effort. But in doing so we make an exchange: we travel a 

 much greater actual distance than 

 if we go " straight up." So, if 

 we wished to raise a 200 pound 

 barrel from the ground to a 

 wagon, we might lift it up directly; 

 but an easier way would be to roll 

 it up an inclined plane (Fig. 181). If the plane were 10 

 feet long and 3M feet from the ground at the higher end, 

 the force required to push the barrel up the plane would 

 be M of 200, or 66% pounds. We roll the 

 barrel 3 times as far as we wish to raise it, 

 but we need exert only J^ of the force. 

 This is the effort required if we exert force 

 parallel with the plane. 



FIG. 181. 

 Inclined Plane. 



FIG. 182. 



The wedge is a form 

 of inclined plane. 



204. The Wedge. The wedge (Fig. 182) is 

 really two inclined planes with their bases placed 



