252 INTRODUCTION TO GENERAL SCIENCE 



186. THE LEVER AND ITS ADVANTAGE 



Any body which moves on a pivot, and has one part farther 

 away from the pivot than another part, is a lever. Any 

 force which is applied to the end farthest from the pivot, 

 will cause the shorter end to exert a greater force. The rela- 

 tion between the resulting force and the applied force is the 

 advantage of the lever. 



The most common example of the lever, which has a large 

 advantage, is the crowbar. If a man desires to pry up a rock, 

 he forces the bar under it and then pushes down on the longer 

 end. Here there is a short length of the bar under the rock; 

 the bar is pivoted on the ground near the rock, and the longer 

 portion of the bar, upon which the force is applied, extends 

 upward. If the part of the bar which is beyond its resting 

 point, or pivot, is one tenth of the longer end, a man who 

 weighs one hundred and fifty pounds can move a rock weigh- 

 ing ten times his own weight, or fifteen hundred pounds. 

 The advantage of the lever, in this case, is ten. 



Sometimes the force is applied to the shorter end of the lever 

 in order to obtain greater velocity at the other end. This 

 is obtained at the expense of force ; that is, the force applied 

 must be greater than the force which is obtained at the longer 

 end. The force multiplied by the numerical measure of the 

 distance through which it acts is equal for both ends of the 

 lever. The bones and joints of animals form levers of 

 this style, and since the muscle acts on the shorter arm of 

 the lever, there is a mechanical disadvantage. That is, the 

 muscle must pull much harder than the force which is being 

 overcome when the animal moves. Thus, in holding at arm's 

 length a weight of twenty pounds, the muscle of the arm is 



