Why Don't 8 Balls Move after Collision?

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The discussion centers on the mechanics of elastic collisions involving eight stationary balls and two colliding balls with velocity v. When the two balls collide with the stationary balls, only the last two balls are ejected with velocity v, rather than all eight moving at velocity v/4 or the last ball moving at velocity 2v. This phenomenon occurs due to the elastic compression of the balls, which effectively transfers the force through the pack without losing kinetic energy, as detailed in a previous thread on Physics Forums.

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Suppose there are 8 balls in line on a horizontal table.
Another 2 balls collide them from left side(both approach together) with velocity v.
The last 2 balls(from right) get knocked off with the same velocity as that of the balls that approached first.Why don't all the 8 balls move with a velocity v/4 or why doesn't only the last ball get knocked off with a velocity 2v?

The collision is elastic.

Something like this:
00 00000000
->
later,
00000000 00
->
 
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This only happens in an ideal case, where there is no loss in kinetic energy. It occurs because of the way the elastic compression of each ball in the pack transfers the force from the colliding balls through the pack of balls to the ejected balls. This was covered in a earlier thread:

https://www.physicsforums.com/showthread.php?t=357358
 
Last edited:

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