90 THE LEVER. 



(2.) The power multiplied by the power-arm equals the 

 weight multiplied by the weight-arm ; or, 

 A (3.) A given power ivill suppoH a weight as many 

 times as great as itself, as the power-arm is times as 

 long as the weight-arm. 



Note. A static law expresses the relation between the power and 

 weight when the machine is in equilibrium. In order that there be 

 motion, one of the products mentioned in the law above must be 

 greater than the other. The lever itself must be in equilibrium 

 before the power and weight are applied. It is to Be noticed that 

 when we speak of the power multiplied by the power-arm, we refer 

 to the abstract numbers representing the power and power-arm. 

 We cannot multiply pounds by feet, but we can multiply the number 

 of pounds by the number of feet. 



171. The Moment of a Force. The moment 

 of a force acting about a given point, as the fulcrum of a 

 lever, is the product of the numbers representing 

 respectively the magnitude of the force and the 

 perpendicular distance between the given point 

 and the line of the force. In the case of the 

 lever represented in Fig. 37, the weight-arm is 8 mm. 

 and the power-arm is 30 mm. Suppose that the power is 

 4 grams, and let the weight be represented by x. Then 

 the moment of the force acting on the power-arm will be 

 represented by (4 x 30 =) 120, and the moment of the 

 force acting on the weight-arm by 82. 



172. Moments Applied to the Lever. We 



sometimes have sey- 

 1H 



eral forces acting 



, 10 I 30 upon one or both 



c | J e\ / arms of a lever, in 



1 1 



or in 



i 8 2 i 



FIG. 40. opposite directions. 



