DYNAMICS, 45 



of dynamics might be written, in which the term 

 Force would not once occur. 



Retarding Forces are here included under the head of 

 Accelerating Forces, as the same force may accele- 

 rate or retard, according as the velocity of the mo- 

 ving body is in one direction or in the opposite. 



87. If a body be continually urged by the same 

 accelerating force, in the same direction, its mo- 

 tion will be uniformly accelerated, or its velocity 

 will increase proportionally to the time. 



The momentary increment of the velocity will be al- 

 ways the same, from the definition of accelerating 

 force ; and as, by the first law of motion, none of 

 the velocity once acquired can ever be lost, it there- 

 fore increases continually at the same rate. 



This is applicable particularly to the case of bodies fall- 

 ing by their gravity to the earth. The descent of 

 these bodies, their velocity being found to increase 

 uniformly, affords a direct and experimental proof of 

 the first law of motion. 



The propositions that follow are all immediately appli- 

 ed to falling bodies ; but they are equally applicable 

 to all rectilineal motions produced by the constant ac- 

 tion of the same force. 



88. If the velocity which a heavy body falling 

 from rest, acquires in the first second of its motion 



3 be 



