8.2 Elastic Collisions in One Dimension
Newton's Cradle

 

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Explanation:
The collisions between the balls in this classical set-up are nearly elastic.  We can use the concepts discussed in Section 8.2 to explain the behavior.  Note that the number of swinging balls never changes during the process. In other words, two balls cannot simply knock out one ball with twice the speed. Can you identify which principle would be violated if that were to occur?
 
Source:
Walter Fendt, Stadtbergen, Germany