SIMPLE MACHINES 



349 



FIG. 191. The chisel as an inclined 

 plane. 



Suppose one is cutting a shaving from a stick of wood with 

 a knife whose blade is six-sixteenths of an inch wide and one- 

 sixteenth of an inch thick on the side opposite its edge ; then this 

 wedge-shaped blade is really an 

 inclined plane. If he is bearing 

 down on the handle of the knife 

 with a pressure of 10 pounds, 

 the blade is exerting a force of 60 

 pounds, less friction, to overcome 

 the cohesion of the wood. So in 

 a chisel (Fig. 191), plane blade, axe, and other cutting tools, we 

 constantly use this simple machine. 



The screw, as we use it on bolts, ordinary wood screws, 

 on the carpenter's bench vise, the screw jack (Fig. 192), and in 

 many other places, is really an application of the inclined plane 

 combined with the lever. Cut a right-angled triangle out of 

 paper, making its base 6 inches long, its altitude i inch. Apply 

 the i -inch side to a pencil and then wrap the paper about the 

 pencil. The hypotenuse of the triangle will make a line like 

 the thread of the screw, but this line in the 

 triangle is a section of an inclined plane. Sup- 

 pose we are turning a nut on a bolt with a 

 wrench (see Fig. 193); the power applied on 

 the handle moves in a circle whose radius we 

 will say is 4 inches. Meantime the head of 

 the bolt has moved 'toward the nut, a distance 

 equal to the space between two turns of the 

 thread. Suppose there are twenty turns of 

 the thread per inch. The distance between 

 threads is then one-twentieth of an inch, which 

 is known as the pitch of the screw. The 

 weight, therefore, has moved one-twentieth of an inch while the 

 power has moved through the circumference of the circle with a 

 radius of 4. The circumference of this circle is twice the radius 

 times 3.1416, or slightly over 25 inches. The power is therefore 



FIG. 192. A screw 

 jack. 



