SIMPLE MACHINES 341 



Notice, however, that he must move his end of the lever a 

 long way down to lift up the weight a short distance. That 

 is because one can never get more energy out of a machine than 

 he puts into it. The weight raised, multiplied by the distance 

 it moves, must equal the power applied, multiplied by the 

 distance it moves. This is a law that will apply to all the 

 machines described below. Now, in the case of the crowbar, 

 both weight and power move through the arcs of circles whose cen- 

 ters are at the fulcrum, and whose radii are the weight arm and 

 the power arm of the lever. The lengths of the weight arm 

 and the power arm are the distances of the ends of these arms 

 respectively from the fulcrum, but these arms are the radii. 

 So we may say that the weight arm multiplied by the weight 

 always equals the power arm multiplied by the power. Suppose 

 that the board of the teeter is 1 1 feet long, and that it weighs 

 2 2 pounds, while the smaller boy weighs 66 pounds and the larger 

 boy 100 pounds, and each sits one-half foot from the end of the 

 plank. Then the fulcrum would have to be 6j feet from the end 

 the smaller boy sits on, for 



396+13=400+9 

 409=409 



If the man in Figure 177 were pressing down on his end of 

 the crowbar with all his weight, say 160 pounds, and this 

 power arm on which he presses were 4 feet long, while the 

 weight arm were 6 inches long, leaving out of consideration the 

 weight of the bar, which may be considered as approximately 

 balancing the element of friction, he could raise a weight of 

 i, 080 pounds. 



Sometimes it is desirable to gain speed of motion in using 

 a lever and sacrifice mechanical advantage. Thus, in striking 

 a blow with the fist in boxing, when the fist is suddenly shot out 

 from the elbow, as the arm is straightened, the fist is the weight. 



