178 SIMPLE MACHINES 



(Fig. 174). The tread of a grindstone and of a weaving 

 loom are examples of third class levers. So is the human 

 forearm (Fig. 277, 355). 



200. Exercises. 



1. To what class of levers do scissors belong? In cutting thick 

 cloth, or a wire, with shears, do you put the cloth, or wire, at the tips 

 of the shears or near the rivet? Why? 



2. What kind of a lever is a pair of sugar tongs? The oar of a boat? 

 The handle of a hoe? A pitchfork? A pump handle? 



3. If you were using a crowbar to roll a log along the ground, how 

 would you use the bar as a lever of the first class? How as a lever of the 

 second class? 



4. If you support a 5 Ib. weight at one end of a bar 3 ft. long by 

 means of a 10 Ib. weight at the other end, where must the fulcrum be 

 placed? 



5. An oar is 7 ft. long, and the handle is 1 ft. from the oarlock. How 

 far is the end of the blade from the oarlock? A boy pulls on the handle 

 with a force of 25 Ibs.; what force does the oar blade exert on the 

 water? Would there be any advantage in having the oar handle 2 ft. 

 from the oarlock? 



6. Why is an oar blade made so broad? Upon what property of 

 water does the blade's usefulness depend? 



7. When a woman is pushing with her toes against the treadle of a 

 sewing machine, where is the power as regards fulcrum and weight? 



What kind of a lever is the treadle in this case? What 

 kind is it when she uses her heel? In which case does 

 she have to do more work? How is the treadle attached 

 to the driving wheel? 



201. Pulleys. In the simplest form of the 

 pulley (Fig. 175) we cannot lift more than 

 FIG. 175. the power we put forth; but we can raise the 

 n y g weight by pulling downward. Such a pulley 

 ifhhl^ght! is used for hoisting the sails of a ship, or for 



