Collision of elastic Bodies. 1 85 



Of the Collision of elastic Bodies. 



296. Although elastic bodies, according to the definition 

 which we have given, must be compressible, we are not hence 

 to infer that they must be so much the more compressible, as 

 they are more elastic. A ball of wool, for example, is not more 

 elastic than a ball of ivory, although it is much more compressi- 

 ble. 



Be this as it may, compressibility seems to be inseparable 

 from elasticity. A body in virtue of its compressibility, changes 

 its figure, when a force is applied to it from without ; and in vir- 

 tue of its elasticity, it tends to recover this figure. But among 

 all elastic bodies, some recover their figure entirely, others only 

 in part. These last are called imperfectly elastic bodies. As to 

 the former, they may resume their figure more or less promptly ? 

 and by very different degrees. But if they are such that, after 

 being struck, they restore themselves according to the same de- 

 grees by which they were compressed, we call them perfectly 

 elastic bodies. In other cases they are denominated simply elastic 

 bodies. We shall here consider only those that are perfectly 

 elastic. 



We observe with respect to perfectly elastic bodies, that in 

 collision, a resistance takes place on the part of the body which 

 has the least velocity, and consequently a compression, and that 

 on this account not only a restoration of the figure follows this 

 compression, but this restoration is itself followed by a new 

 change of figure directly contrary to the first. To this succeeds 

 another, which reduces the body to the figure first given by the 

 compression, and so on. In this way the parts of each body 

 have, with respect to their centre of gravity, a vibration, or mo- 

 tion backward and forward ; since the parts tend to return to 

 their first figure by a motion which goes on increasing, and thus 

 carries them beyond their former position. These changes of 

 figure, which alternate with each other, are sensible in several 

 elastic bodies, when struck, and particularly in those that are of 

 the sonorous class. 



Mech. 24 



