1912^ Fundamental Bases of Dynamics 75 



or slightly less than half an ounce avoirdupois. Such a force 

 has been called a poundal. 



By equation (1) putting W = 1, gr = 32.174, we have 

 Wi 1 



g^ 32.174 



In this equation, W^ represents the attraction of the earth on 

 the metal piece, called a pound weight, at any place where the 

 acceleration is g^. Reasoning as before, the force W^ acting 

 freely on the metal piece will cause it to acquire in one second 

 a velocity of gi feet per second ; hence the force 



Wi 1 



— = pound = 0.031081 pound, 



g^ 32.174 



will produce in the body a velocity of one foot per second at the 

 place considered. The unit of force, the poundal, in the absolute 

 system, is thus constant and it may be thought of as a pull or a 

 push of a little less than i/o ounce. Let the student realize that 

 in the absolute system, in connection with the formula Y = m a, 

 that m is expressed by the same number that represents the 

 weight in pounds of the body as found (anywhere) by an equal- 

 armed balance, and that F is expressed in poundals, where a 

 poundal is 0.031081 pound force. 



An answer in poundals can be readily expressed in pounds 

 of force by dividing by 32.174 or multiplying by 0.031081. In 

 Technical Mechanics, it is well to give the absolute system in 

 an Appendix, for although the engineer student will not use it 

 in his ordinary work, still a slight study of the absolute system 

 will enable one to read valuable works that otherwise might 

 offer difficulties. 



The same remarks apply to the French C. G. S. system, which 

 may be analyzed as above. It is well to bear in mind that in any 

 absolute system, the lever balance always measures mass, 

 whereas in the engineers' system the spring balance measures 

 the attraction of the earth (weight). 



Chapel Hill, N. C. 



