fiOS On Machines in GeJicraL 



This law consists in this, that every body which changes 

 its state of repose or uniform and rectilinear motion, 

 never does sd except by the influence or action of some 

 other body, upon which it impresses, at the same lime, a 

 <}iianlity of motion eqnal and directly opposite to that 

 which it receives from it ; that is to say, that the velocity 

 it assumes the instant afterwards is the force resulting from 

 that which this other body impresses upon it, and from that 

 which it would have had without this last force. Every 

 body therefore resists Its change of state ; and this resistances^ 

 which is called vis inerticey is always equal and directly op-; 

 posite to the quantity of motion it receives, i, e. to the 

 quantity of motion which combined with that which it 

 had immediately before the change, produces, as the result, 

 the quantity of motion which it should really have im- 

 mediately afterw-ards. This is also expressed by saying, that 

 in the reciprocal action of bodies, the quantity of mo- 

 tion lost by the one is always gained by the others, in the 

 fame time and in the same ratio. 



Sfxond Law. — IVhen two hard bodies act upon each 

 other, hy shock or pressure, i. e. in 7'irtue of their inpene- 

 trabiliti/, their relative velocity y immediately after the reci- 

 procal action, is always juilL 



In fact, we constantly observe, that if I wo hard bodies 

 give a shock to each other, their vefocities, immediately 

 after the shock, estimated perpendicularly to their commoft 

 surface at the point of contact, are equal, in the same way 

 as if they were drawn by inextensihle wires, or pushed 

 by incompressible rods ; their velocities, estimated in the 

 ratio of this wire or rod, would necessarily be equal : whence' 

 h follows that their relative velocity, e. e, that by. which 

 they approach or recede from each other, is in every case 

 null at the hrst instant. 



From these two principles it is easy to draw the laws of 

 the shock of hard bodies, and consequently to conclude the 

 two other secondary principles, the use of which is continual 

 in mechanics, viz. 



1. That the intensity of the shock, or of the action which 



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