75 



LECTURE VIII. 



ON COLLISION. X 



Having inquired into the laws and properties of tlie motions and rest of 

 single bodies, under the operation of one or more forces, and into the equili- 

 brium of these forces, in different circumstances, we are next to examine 

 some simple cases of the motions of various moveable bodies acting recipro- 

 cally on each other. In all problems of this kind, it is of importance to re- 

 collect the general principle already laid down, respecting the centre of in- 

 ertia, that its place is not affected by any reciprocal or mutual action of the 

 bodies constituting the system. 



Whenever two bodies act on each other, so as to change the direction of 

 their relative motions, by means of any forces which preserve their activity 

 undiminished at equal distances on every side, the relative velocities with 

 which the bodies approach to, or recede from each other, will always be 

 equal at equal distances. For example, the velocity of a comet, when it 

 passes near the eajth in its descent towards the sun, is the same as its velocity 

 of ascent in its return, although, at different distances, its velocity has under- 

 gone considerable changes. In this case, the force acts continually, and at- 

 tracts the bodies towards eacli other; but the force concerned in collision, 

 when a body strikes or impels another, acts only during the time of more or 

 less intimate contact, and tends to separate the bodies from each other. ' 

 When this force exerts itself as powerfully in causing the bodies to separate, 

 as in destroying the velocity with which they meet each other, the bodies 

 are called perfectly elastic : when the bodies meet each other without a re- 

 action of this kind, they are called more or less inelastic. Ivory, metals, 

 and elastic gum, are highly, and almost perfectly elastic : clay, wax, mixed 

 with a little oil, and other soft bodies, are almost inelastic : and tlie effects 



