494 



GENERAL SCIENCE 



Pulleys. Pulleys are also used as machines for saving power. 

 Barrels, iron bars and other heavy material are often lifted into 

 place by the aid of the pulley. 



The illustration will show that if there were no friction, the 

 amount of force needed to lift the weight would be equal to the 

 weight divided by the number of ropes supporting the weight. 

 (Find out by experiment the distance the weight travels as com- 

 pared with the distance the force travels when pulleys are used.) 



10 Ibs. 



FIG. 374. How many ropes are connected with the pulley pulling the weight 

 in each case? What does the number of ropes divided into the total weight 

 equal in each case? 



The Wheel and Axle. The wheel and axle is, in reality, a lever, 

 the central axle being the fulcrum, the radius of the large wheel 

 the power arm, and the radius of the small wheel the weight arm. 

 The ice cream freezer, bread mixer, wringer, coffee mill, door knob, 

 etc., are examples of the wheel and axle type of lever. 



The Screw. Another useful appliance is the screw, such as the 

 jack screw, the bolt and nut, the faucet, the meat chopper, the screw, 

 the clamp, the screw on the cover of a preserving can, etc. 



