^4 



WELLS'S NATURAL PHILOSOPHY. 



What are the 

 relative posi- 

 tions of the 

 power, fulcrum 

 and weight in 

 the three kinds 

 of levers i 



£" 



Fig. 61. 

 I 



What are ex- 

 amples of le- 

 vers of the 

 first class t 



Wc 



^ir 



201. In the first class the fulcrum is be- 

 tween the power and the weight ; in the sec- 

 ond class, the fulcrum is at one end of the 

 lever, and the weight is between the fulcrum 

 and the power ; in the third class, the fulcrum 



is at one end of the lever, and the power is between the 



fulcrum and the weight. 



Fig. 61 represents the three classes of 

 ^vers, numbered in their order, 1. 2, 3. 

 P is the power, W the weight, and F tho 

 fulcrum. 



A crowbar applied to 

 elevate a stone, is an ex- 

 ample of a lever of tho 

 first kind. In Fig. 62, 

 which represents a lever of this class, a 

 indicates the fulcrum which suppports the 

 bar, b the power appUed by the hand at 

 the end of the longest arm, and c tho 

 weight, or stone, raised at the end of the 



short arm. A poker appUed to stir up the fuel of a grate i3 a lever of th© 



first class, the fulcrum being th« 

 bars of the grate ; the break, or 

 or handle of a pump, is also a fa- 

 miliar example. Scissors, pin- 

 cerg, etc., are composed of two 

 levers of the first kind, the ful- 

 crum being the joint, or pivot, 

 and tho weight tho resistance 

 of the substance to be cut, or seized. The power of the fingers is appUed 

 at tho other end of the levers. 



What is the 202, A lever will be in equilibrium, when 

 brhim^ o?"the t^s power and the weight are to each other 

 lever? inversely as their distances from the fulcrum. 



Thus, if in a lever of tho first class the power and the weight are equal, 

 and are required to exactly balance each other, they must be placed at 

 equal distances from tho fulcrum. If the power is only half the weight, it 

 must be at double the distance from the fulcrum; if one third of tho 

 weight, three times the distance. If we suppose, in Fig. 62, c to represent 

 a weight of 300 pounds, placed two feet from the fulcrum a, and b a power 

 of 100 pounds placed six feet fi-om a, then c and 6 will be in equilibrium, 

 for (300X2) = (100X6). 



203. When the weight and lengths of the two arms 



Fig. 62. 



