MACHINES 



63 



FIG. 60. The Wedge. 



long and 2 inches thick at the thickest part, how far will 

 the force have moved when the wedge has been driven 

 into the block? How does the 

 amount of work done compare 

 with the force applied ? 



The wedge may be considered 

 as an inclined plane which is 

 forced by blows between two 

 resistances in such a way as to 

 separate them. 



Mechanical Advantage. It 

 is often possible with the aid of 

 a machine to overcome a certain 

 resisting force by applying a 

 much smaller force. The ratio of the resistance overcome 

 to the force applied is called the mechanical advantage 

 of the machine. Thus if both arms of a lever of the 

 first class are of the same length, the mechanical advan- 

 tage will be 1. If the force arm is two times as long as 

 the resistance arm the mechanical advantage will be 2, 

 while if the force arm is one half as long as the resistance 

 arm, the mechanical advantage will be one half. 



Suppose the force arm of a lever of the first class to be 

 four feet long and the resistance arm to be one foot long, 

 then a force of one pound will overcome a resistance of four 

 pounds ; but it will be noticed that the acting force moves 

 four times as far as the resisting force and also four times 

 as fast. We can sacrifice speed and distance to gain force 

 or we can sacrifice force to gain speed or distance. 



Efficiency in Machines. Not all the force applied 

 to a machine is effective in doing useful work. Some 

 work must be done in overcoming friction in the machine 

 and in moving parts of the machine itself. 



