SIMPLE MACHINES 



181 



wasted by friction and nine parts results in useful work. 

 When an object is lowered by a machine, the efficiency 

 of the machine is usually more than 100 per cent, because 

 friction is resulting in useful work by helping to keep 

 the object from descending too fast. Friction is useful 

 in checking the speed of street cars and trains going 

 down long grades and also for stopping them. 



122. The Lever. The lever is a rod free to turn about 

 a point. The fulcrum is the point around which the 

 lever turns. A seesaw is one kind of a lever: if the two 

 persons on it are of equal weight, they take positions 

 which are the same distance from the point about 

 which the seesaw turns. If two persons of unequal 

 weight get on a seesaw, the one who is the heavier will 

 take a position closer to the fulcrum than the one of less 

 weight. If a person whose weight is 100 pounds takes 

 a position 6 feet from the fulcrum and a person whose 

 weight is 75 pounds takes a position 8 feet from the 

 fulcrum, they will balance on the seesaw. The reason 

 that they will balance is because 100 X 6 is equal to 



4|0 3|0 2|0 HO 



LEVER OF THE FIRST CLASS 



75 X 8. If a person weighing 50 pounds is 10 feet from 

 the fulcrum and a person weighing 100 pounds is 5 feet 

 from the fulcrum, they will balance, because 50 X 10 is 

 equal to 100 X 5. 



Balance a meter stick as shown in the illustration, and 



