38 INTRODUCTION TO DYNAMICS. [65. 



i -r .mime moving with an acceleration of i cm. per second per 

 second. This unit force is called a dyne. 



The definition is sometimes expressed in a slightly different 

 form.* We may say the dyne is the force which, acting on a 

 gramme uniformly for one second, would generate in it a velocity 

 of i cm. per second ; or would give it the C.G.S. unit of acceler- 

 ation ; or it is the force which, acting on any mass uniformly for 

 one second, would produce in it the C.G.S. unit of momentum. 



That these various statements mean the same thing follows 

 from the fundamental formulae F=mj, jvt, if F, m, t, v,j be 

 expressed in C.G.S. units. 



65. In the F.P.S. system, the unit of force is the force of a 

 mass of i Ib. moving with an acceleration of i ft. per second 

 per second. It is called the poundal. 



66. The dyne and the poundal are called the absolute, or 

 scientific, units of force. 



To find the relation between these two units, let x be the 

 number of dynes in the poundal ; then we have 



hence, just as in Art. 58, 



^=13825.3; 



i.e. i poundal = 13825.3 dynes, and I dyne =0.000072331 

 poundals. 



67. Another system of measuring force, the so-called gravi- 

 tation (or engineering) system, is in very common use, and must 

 here be explained. 



Among the forces of nature the most common is the force of 

 gravity, or the weight, i.e. the force with which any physical 

 body is attracted by the earth. As we have convenient and 



* J. D. EVERETT, C.G.S. system of units, 1891, p. 23, 24. ^ 



