298 



HOW MACHINES LIGHTEN LABOR 



many cases, it is preferable to exert a force of 30 pounds, for 

 example, over the distance CA than a force of 120 pounds over 



the shorter distance BA. 



Prop the board so that the 

 end A is 2 feet above the 

 table level ; that is, arrange 

 the inclined plane in such a 

 way that its length is twice 

 as great as its height. In 

 that case the steady pull on 

 the balance will be one half 

 the weight of the roller ; or a 

 force of 6 pounds will suffice 

 to raise the 12-pound roller. 

 The steeper the incline, the 

 greater is the force necessary 



FIG. 163. Primitive man tried to lighten . . . . .., 



his task by balancing his burden. to raise a weight ; whereas if 



the incline is small, the neces- 

 sary lifting force is much reduced. On an inclined plane whose 

 length is ten times its height, the lifting force is reduced 

 to one tenth the weight of 

 the load. The advantage of 

 an incline depends upon the 

 relative length and height, 

 or is equal to the ratio of the 

 length to the height. 



Application. By the use 

 of an inclined plank a strong 

 man can load the l6o<D-pOUnd FlG - 164- Less force is required to raise 

 ill c the roller along the incline than to raise it 



bowlder on a wagon. Sup- to A ^^ y 



pose the floor of the wagon 



is 2 feet above the ground, then if an 8-foot plank is used, 400 



pounds of force will suffice to raise the bowlder. But the man 





