SIMPLE MACHINES 189 



is widely used. Lumbermen use both iron and wooden 

 wedges to split logs. An iron wedge 8 inches long and 

 one inch thick at the large end, has a mechanical ad- 

 vantage of 8. If this wedge is struck with a five-pound 

 hammer descending at a speed of 100 feet per second, it 

 will lift a weight of 4,000 pounds. (The force of the 

 hammer is 100 X 5 or 500 pounds; 500 X 8 = 4,000.) 



126. The Screw is a modified inclined plane. The 

 threads of a screw may be thought of as an inclined plane 

 wrapped around a rod. Since a 

 lever of some kind is used to 

 turn the screw, the whole ma- 

 chine may be regarded as a com- 

 bination of the inclined plane 

 and the lever. When the screw 

 is turned once around it moves SHOWING THE PITCH OF A SCREW 



a vertical distance equal to the distance between the 

 tops of two adjoining threads; this distance is the space 

 between two adjoining threads plus the thickness of the 

 thread. This vertical distance is called the pitch of the 

 screw. So when the lever of a screw makes one complete 

 revolution, the object on the screw is moved a distance 

 equal to the pitch of the screw being used. Hence, the 

 mechanical advantage of a screw is 

 the result obtained by dividing the 

 distance moved by the force used in 

 making one complete revolution, 



SQUARE THREADS AND V by the pitch of the screw. If the 



THREADS OF A SCREW . , .. . " V ' ... , i i 



pitch of a screw is J inch and the 



lever 2 feet, the mechanical advantage is 2X2XI2X 

 3.1416 -T- .5, or 301.5. If a force of 100 pounds were ap- 

 plied at the end of the lever, a weight of 301.5 X 100, or 

 30,150 pounds, could be lifted, if there were no friction. 



