CHAPTER THREE 



CONSERVATION OF ENERGY 



SECTION 10. Levers. 



How a big weight can be lifted with a little force; how one 

 thing moving slowly a short distance can make another move 

 swiftly a long distance. 



Why can you go so much faster on a bicycle than on foot ? 



How can a man lift up a heavy automobile by using a 

 jack? 



Why can you crack a hard nut with a nutcracker when 

 you cannot crack it by squeezing it between two pieces of 

 iron ? 



" Give me a lever, long enough and strong enough, 

 and something to rest it on, and I can lift the whole 

 world," said an old Greek philosopher. And as a philos- 

 opher he was right; theoretically it would be possible. 

 But since he needed a lever that would have been as 

 long as from here to the farthest star whose distance 

 has ever been measured, and since he would have had 

 to push his end of the lever something like a quintillion 

 (1,000,000,000,000,000,000) miles to lift the earth one 

 inch, his proposition was hardly a practical one. 



But levers are practical. Without them there would 

 be none of our modern machines. No locomotives 

 could speed across the continents ; no derricks could 

 lift great weights ; no automobiles or bicycles would 

 quicken our travel ; our very bodies would be com- 

 pletely paralyzed. Yet the law back of all these things 

 is really simple. 



You have often noticed on the see-saw that a small 

 child at one end can be balanced by a larger child 

 at the other end, provided that the larger child sits 



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