DYNAMICS. 43 



SECT. IV. 



OF MOTION EQUABLY ACCELERATED OR RE- 

 TARDED. 



85. W HEN the motion of a body varies either 

 in direction or velocity, as this cannot arise from 

 any thing in the hody itself, it must he ascrihed to 

 the action of an external cause or force. 



The simplest case is, when the direction remains the 

 same, and when the velocity only varies. 



86. When the velocity of a hody changes, the 

 cause of that change is called an accelerating or 

 retarding force, and is measured by the change of 

 the velocity produced in an indefinitely small por- 

 tion of time, or by the increment of the velocity 

 divided by the increment of the time. 



If F be the accelerating force, v the momentary in- 

 crement of the velocity, i that of the time, 



Hence also v = F t, and /=_.'. 



r 



It 



