40 JUNIOR GRADE SCIENCE 



The perpendicular distances from the fulcrum to the lines of actions 

 of forces acting upon a lever, are known as the arms of the lever. In 



rm Zl Arm 



Weight I 



FIG. 30. Terms used in connection with levers. 



Fig. 30 the distance AC is the arm at one end of which the "weight" 

 acts, and BC is the arm at one end of which the " power " acts. 



Principle of the lever. It is easy to show by experiment, that 

 when a lever is in equilibrium the following equation holds good :' 



force on x distance _ force on distance 



one side from fulcrum other side x from fulcrum. 



This principle of moments applies to all levers, so that all that need 

 be remembered when considering the action of a lever of any kind, 

 are the forces working in one direction and their distances from the 

 fulcrum, compared with the forces or resistances which oppose them 

 and the distances of these from the fulcrum. 



Moments. Refer to the diagram (Fig. 31), where F represents the 

 point of support, or fulcrum, of a lath or other straight lever, and M l 

 is a weight at a distance AF in equilibrium with a smaller weight J/ 2 , 

 at a greater distance FB. 



The force acting at A is the weight of MI, acting vertically down- 

 wards ; and the force at B is the weight of M 2 , acting in the same 



Vf, *, 



FIG. 31. To illustrate moments of forces. 



direction. Each force tends to turn the lever in a particular direction, 

 and this turning effect is called the moment of the force. The moment 

 of the force acting vertically downwards at A is the product of the force 

 equal to the weight of M l into the distance AF, which, as the diagram 

 shows, is measured at right angles to the direction in which the force 

 acts. Similarly, the moment of the force equal to the weight of M.,, 

 about the point F, is equal to the product of this force and the perpen- 

 dicular distance BF. 



