70 



WELLS'S NATURAL THILOSOPHY. 



The quantity of motion will remain unchanged, the one having gained as 

 much as the other has lost ; so that the two, if equal, will have half the ve- 

 locity after collision that the moving one had when alone. Fig. 38 represents 

 two balls of clay, E and D, non-elastic, of equal-weight, suspended by strings. 

 If the ball D be raised and let fall against the ball E, a part of its motion will 

 be communicated to E, and both together will move on to e d. 

 When two I'lS- If ■^^'e suspend two balls, A and B, Fig. 39, of some 



eliistic bodies elastic substance, as ivory, and allow them to fall with equal 

 come into col- i ./> -i 



lision, wliatoc- masses and velocities from the points X and Y on the arc, 



""^^^ they will not come to rest after collision, but will recede 



Fig. 39. 



What occa- 

 Gions the dif- 

 ference in the 

 results of the 

 collision of 

 eListic and non- 

 elastic bodies? 



from each other with the same velocity which each 

 had before contact. 



The reason of this movement in 

 highly elastic bodies, contrary to 

 what takes place in non-elastic 

 bodies, is this: the elastic sub- 

 stances are compressed by the force 

 of the shock, but instantly recover- 

 ing their former shape in virtue of their elasticity, 

 they spring back, as it were, and react, each giving 

 to the other an impulse equal to the force which 

 caused its compression. 



Suppose the ball A, however, to strike upon the 

 ball B at rest ; then, after impact, A will remain at 



rest, but B will move on with the same velocity as A had at the moment of 

 contact. In this case the reaction of elasticity causes the ball A to stop, and 

 the ball B to move forward with the motion which A had at the instant of 

 contact. 



Fig. 40. 



~Mr 



CXXDOOOO 



^ J3 C n EF G- 



The same fact may be illustrated 

 by suspending a number of elastic 

 balls of equal weight, as represented 

 in Fig. 40. If the ball H be drawn 

 out a certain distance, and let fall 

 upon G, the next in order, it vnii 

 communicate its motion to G, and 

 receive a reaction from it, which will 

 destroy its own motion. But the 

 ball B can not move without moving 

 F; it will, therefore, communicate 



the motion it received from G to F, and receive from F a reaction which will 

 stop its motion. In like manner, the motion and reaction are received by each 

 of the balls E, D, C, B, A, until the last ball, K, is reached ; but there being 

 no ball beyond K to act upon it, K will fly off as far from A^ as H was 

 drawn apart from G. 



