RESISTANCE OF MATERIALS 



71 



of the external forces must equally be zero. We apply this 

 theorem to simple machines. 



Fio. 64. 



(1) Levers. A lever is a rigid bar, movable round affixed 

 point called the fulcrum (A). The resistance, Q, to be overcome, 



is a force or weight applied at the 

 .p extremity B of the lever and the 



I rf power P is applied at the other 



I ~"7 Q extremity C (fig. 84). This type of 



* lever is called a lever of the first 



order. But the resistance can be at 



7 - the centre of the lever when it is said 



\Q \ p to be of the second order, or, finally, 



i _ the power can be applied at the centre 



j iL ^ when it is said to be of the third order 



In (fig. 85). Scissors, tinman's snips, 



blacksmiths' tongs, turners' tools, 

 pruning shears, etc., are of the first 

 order. Sheet iron shears, the bakers* 



knife, nutcrackers, oars or sculls, the handles of pumps, wheel- 

 barrows, pedals of pianos, etc., are of the second order, and spades, 

 coal-tongs, the treadle of the knife-grinder or sewing machine, and 

 the majority of the numerous levers found in the structure of 

 animals are of the third order. 



The distances between the fulcrum and the positions of the 

 power and of the resistance constitute the arms of the lever. In 

 the lever CAB (fig. 86), the arms are Ap and Ar, and not AC and 

 AB. The resultant R will be applied to the fulcrum, A. 



Fio 85 



P.O. 83. 



