60 FIRST YEAR COURSE IN GENERAL SCIENCE 



FIG. 20. THE LEVER 



FIG. 21. PULLEYS 



FIG. 22. WHEEL AND AXLE 



FIGS. 20-23. SIMPLE MACHINES 



Two advantages are derived from the use of 

 simple machines: the power can be applied in various 

 directions; and by the right adjustment, a small 

 power can move a weight greater than itself. For 

 example, to take a stone from its place requires 

 lifting (that is, pulling upwards) by a force equal 

 at least to the weight. By means of a stiff bar 

 resting on a support, called the fulcrum, between 

 the stone and the workman, he can move the stone 

 by pushing down instead of lifting. 



1. Why is it easier to push down than to pull 

 FIG. 23. THE SCREW up? 2. If the fulcrum is 1 ft. from the stone and 

 P is applied 3 ft. from the fulcrum, the stone 



can be moved by applying only \ as many pounds of power as the weight 

 of the stone. The distances of P and W from the fulcrum are as 3 to 1, 

 which makes it possible for a given power to lift a weight three times as 

 great as the power itself. If the stone weighs 450 lb., what power would 

 be needed to move it? 



