14 MECHANIC S, 



INERTIA OF MOVING BODIES, OR MOMENTUM. 



Momentum is the inertia of a moving body. When a 

 force is applied to a heavy body, its motion is at first slow ; 

 but the little momentum it thus acquires, added to the ap- 

 plied force, increases the velocity. This increase of velocity 

 is of course attended with increased momentum, which 

 again, added to the acting force, still further quickens the 

 speed. For this reason, when a steam-boat leaves the pier, 

 and its paddle-wheels commence tearing through the wa- 

 ter, the motion, at first slow, is constantly accelerated un- 

 til the increasing resistance of the water becomes equal 

 to the strength of the engine and the momentum.* Were 

 it not for the momentum of moving bodies (inertia exist- 

 ing), no speed ever could be given to any heavy body, as 

 a carriage, boat, or train of cars. 



The chief danger in fast riding, or fast traveling of any 

 kind, is from the momentum given to the traveler. If a 

 rail-way passenger should step from a car when in full mo- 

 tion, he would strike the earth with the same velocity as 

 that of the train ; or if the train at thirty miles an hour 

 should be instantly stopped, the passengers would be 

 pitched forward with a swiftness equal to thirty miles an 

 hour. When a horse suddenly stops, the momentum of 

 the rider tends to throw him over the horse's head. When 

 a wagon strikes an obstruction, the driver falls forward. 

 A case in court was once decided against the plaintiff, who 

 claimed that the defendant had driven against his wagon 

 with such force as to throw the plaintiff to a great distance; 

 but the fact was shown that by such momentum he him- 

 self must have been driving furiously, and not the defend- 

 ant, and he lost his suit. 



* In ordinary practice, this is not strictly correct, as /Hc*<io;i will make 

 eomc difference. This inlluencc will be more particularly considered on 

 a subsequent paj^e. Its omission here does not at all alter iha principle 

 under consideration. 



