SIMPLE MACHINES. 79 



a nail into a board or a beam requires far more work. 

 The force necessary to drive the nail into the wood may, 

 in the case of a nail of average size, be taken as 1 00 kgr ; 

 if the depth to which the nail is to be driven in is 

 4n ( = o m -04), the work required is 100 x 0'04 = 4 

 kilogrammetres. This work cannot be performed by 

 the hand alone, because it is incapable of exerting a 

 pressure of 100 kgr . We use for this reason a hammer, 

 to which the muscular force of the arm gives a certain 

 velocity, that is a certain amount of accumulated 

 work. If the blows were delivered at a distance of 

 O m '25, with a force of 2 kgr , the accumulated work would 

 be 2 x 0*25 = 0*5 kilogrammetres, when the hammer 

 reaches the nail. The nail prevents the onward motion 

 of the hammer, and the work is now expended in over- 

 coming the resistance of the wood. Each blow does 

 0*5 kilogrammetres of work; hence eight blows will 

 be required to drive the nail to the required depth. 



tin a similar manner many cases occur in which work 

 be performed can only be done with the help 

 of certain auxiliary contrivances. These are called 

 simple machines, or mechanical powers ; and their func- 

 tion is to convert one form of work into another form. 

 None of these machines can generate work, nor increase 

 it; on the contrary, they all involve a certain loss of 

 work, which is expended in overcoming the resistance 

 which the force of friction opposes to their motion. The 

 advantage of a machine consists in the conversion of 

 work which is given in an unavailable form into an 

 available form. 



An example of such a simple machine is the Wheel 

 md Axle, essentially a combination of inseparably con- 



