MECHANICAL WORK 115 



the gradual application of power. You apply power 

 gradually to a nutcracker so that the shells will not fly on 

 the floor and the kernels be crushed. A wheelbarrow is 

 another lever of this class. 



In levers of the third class it is the applied power which 

 is between the fulcrum and the resistance (see Fig. 54). 



FIG. 53. A lever of the second FIG. 54. A lever of the third 

 class. class. 



This class of lever you can illustrate by straightening out 

 your arm and then raising your hand to your shoulder. 

 Your elbow is the fulcrum, and the weight of your forearm 

 is the resistance. The power is applied at that point in 

 front of the elbow at which the tendons of the muscle of 

 your upper arm (biceps) are attached to a bone of the fore- 

 arm. You lift the forearm by contracting the upper arm, 

 but the power is applied just in front of the elbow. 



QUESTIONS 



1. Explain the principle of the inclined plane. 



2. Why is it easier to lift a thing with a windlass than by hand? 



3. If a two-hundred-and-forty-pound stone were on the short end 



of a lever, eight inches from the fulcrum, and if the long end 



