244 Our Surroundings 



The Lever. The lever is a rigid bar arranged to turn on 

 a fixed support, or axis, called the fulcrum (F). By its use, 

 a limited force, or effort, may overcome a large resistance or 



1 weight. The distance on the 



bar from the fulcrum to the 

 I4CM - .j point of support of the weight 



is called the weight arm, or 

 resistance arm of the lever. 

 , , The distance from the fulcrum 



to the point where the effort is 

 A BALANCED LEVER v j i ** * 



Why does the small weight balance applied IS known as the force 



the large one? Qrm Qr e ff on arm f the fc ver> 



The force used in a lever or other machine is called the effort 

 (E), whether it is the pressure of water, the pull of gravity, or 

 any other force. The resistance to be overcome is the weight (W). 



Experiment to Illustrate the Lever. First balance a meter 

 stick by supporting it at its middle point. This support is the 

 fulcrum. With a cord hang a 500-gram weight on the meter 

 stick from a point 14 centimeters from the fulcrum. This is 

 the weight, or resistance. Then find a point on the other side 

 of the fulcrum at which a 200-gram weight, the effort, will 

 just balance the 500-gram weight. This point will be 35 centi- 

 meters from the fulcrum on the other side. A meter stick so 

 arranged is a lever. 



Notice that 500, the weight, multiplied by 14, the distance 

 from the fulcrum, exactly equals 200, the effort, multiplied by 

 35, its distance from the fulcrum. Both products are 7,000. 

 This illustrates the general truth that when two different weights 

 are just balanced, the product of one weight multiplied by its 

 distance from the fulcrum is always equal to the product of the 

 other weight multiplied by its distance from the fulcrum. In 

 other words, the weight multiplied by the weight arm equals the 

 effort multiplied by the effort arm. This is known as the principle 

 of moments and is applied in common forms of scales. 



Classes of Levers. Levers are divided into three groups, 

 known as levers of the first, second and third classes. The class 

 to which a lever belongs is determined by the relative location of 



