114 THE SCREW. 



15. A book-binder has a press; the threads of its screw are I in. apart-, 

 the nut is worked by a lever which describes a circumference of 

 8 ft. How great a pressure will a power of 15 Ibs. applied at the end 

 of the lever produce, the loss by friction being equivalent to 240 Ibs. ? > 



16. A screw has 11 threads for every inch in length. If the 

 lever is 8 inches long, the power, 50 pounds, and friction is -J of the 

 energy used, what resistance may be overcome by it ? 



17. A screw with threads 1] in. apart is driven by a lever 4* ft. 

 long ; what is the ratio of the power to the weight ? (See Appendix A. ) 



18. How great a pressure will be exerted by a power of 15 Ibs. 

 applied to a screw whose head is one inch in circumference and 

 whose threads are inch apart ? 



19. At the top of an inclined plane which rises 1 ft. in 20 is a wheel 

 and axle. Radius of wheel =2 ft. ; radius of axle 4|- in. What load 

 may be lifted by a boy who turns the wheel with a force of 25 Ibs. ? 



20. The crank of an endless screw whose threads are an inch 

 apart describes a circuit of 72 inches. The screw acts on the 

 toothed edge of a wheel 60 inches in circumference. On the axle 

 of this wheel, which is 10 inches in circumference, is wound a cord 

 which acts upon a set of pulleys, 3 in each block. The effect of the 

 pulleys is exerted upon the wheel of a wheel and axle. The diam- 

 eters of the wheel and of the axle are 4 ft. and 6 inches respec- 

 tively. What weight on the wheel and axle may be lifted by a 

 force of 25 Ibs. at the crank, allowing for a loss of -*- by friction ? 



21. An endless screw which is turned by a wheel 10ft. in circum- 

 ference acts upon a wheel having 81 teeth ; this wheel has an axle 

 18 inches in circumference ; the power is 75 Ibs. Find the value of 

 the weight that may be suspended from the axle. 



22. In moving a building the horse is attached to a lever 7 feet 

 long, acting on a capstan barrel 11 inches in diameter ; on the barrel 

 winds a rope belonging to a system of 2 fixed and 3 movable pul- 

 leys. What force will be exerted by 500 pounds power, allowing 

 | for loss by friction ? 



Recapitulation. To be amplified by the pupil for 

 review. 



WFnPF J DEFINITION. 



WhUbt. -j TWO USES AND THE LAW FOR EACH. 



( DEFINITION. 

 SCREW. -I LAW. 



( ENDLESS SCREW ; ITS ADVANTAGES ; RELATION OF P TO W. 



COMPOUND MACHINES ; RELATION OF P TO w. 



(DEFINITION. 



FRICTION.^ CAUSE. 



EIGHT FACTS. 



