LAW OF MACHINES 175 



any energy. They simply make it possible for us to apply 

 force in a convenient direction, or at a convenient place. 

 Or they make it possible for us to exchange a small force 

 exerted through a considerable distance for a much larger 

 force exerted through a correspondingly shorter distance. 

 Thus, we give the head of a screw driver one complete 

 turn in order to get the screw to move forward only the 

 distance between two successive threads; but the force 

 we exert in overcoming the cohesion of the wood is enor- 

 mously greater than that which we ap- 

 ply to the head of the screw driver. 

 Again, we probably could not draw a 

 nail out of a board by pulling with all 

 our might, yet by the use of a claw 



FIG. 167. 



hammer (Fig. 167) we need to put forth 



only a small effort to do the work. But A claw hammer is a 



machine. 



we must remember that in using ma- 

 chines we are making an exchange, not getting something 

 for nothing. 



The exchange we make in using a sewing machine is the 

 opposite of that which we make in the screw and the 

 hammer. Sewing by hand is hard, not because it requires 

 much strength, but because it is slow. With a sewing 

 machine the seamstress can exchange her strength for 

 greater speed. 



The law of machines is merely the statement of the 

 exchange we make in every machine. If we multiply 

 the power exerted (stated in weight units) by the dis- 

 tance through which the source of the power moves, 

 the product is just equal to the product obtained when 

 we multiply the resistance overcome, or the weight 



