182 GENERAL SCIENCE 



let a mass of 200 grams be hung by a thread from a point 

 30 cm. from the fulcrum. Then let a point be found on 

 the opposite side of the fulcrum at which a weight of 150 

 grams will just balance the 200 grams This point will 

 be found to be 40 cm. from the fulcrum. We see the 

 reason at once, because the product of 200 X 30 is equal 

 to the product of 150 X 40. 



The seesaw and the meter stick, when used as described 

 above, are levers. The important things to keep in mind 

 about a lever are: (i) the large weight, which may be 

 called the resisting force; (2) the small weight, which may 

 be called the acting force; (3) the fulcrum, about which the 

 lever turns; (4) the distance from the fulcrum to the weight, 

 which is called the weight arm; and (5) the distance from 

 the fulcrum to the force, which is called the force arm. 



The law of levers is: The force multiplied by the length 

 of the force arm is equal to the weight multiplied by the 

 length of the weight arm. 



Force X force arm = weight X weight arm. 

 For instance, 10 g. with a force arm of 10 cm. will balance 

 25 g. with a weight arm of 4 cm: 10 X 10 = 25 X 4. 

 That is, the moment of the acting force is equal to the 

 moment of the resisting force. The moment of a force is 

 that which is trying to produce rotation around a point. 

 When the two opposing moments are equal there will be 

 no motion and the lever will balance. 



The mechanical advantage of a lever is (i) the ratio of 

 the weight to the force, or (2) the ratio of the force arm 

 to the weight arm, or (3) the ratio of the distance the 

 force moves to the distance the weight is moved. 

 d) (2) (3) 



W F. arm F. distance , . , , 



- = - - = - - = mechanical advantage. 



F W. arm W. distance 



