In an elastic collision, 2 processes can be said to take place as mentioned here in the accepted answer: Why does a billiard ball stop when it hits another billiard ball head on?

First by the interacting force and then when the deformation is undone.

But why would the the latter process ('expanding back to original shape') cause changes in the energy/velocity of the same whole body ? Wouldn't it only cause changes in the local area where the deformation had taken place ?

  • $\begingroup$ If there is deformation, the collision is not elastic $\endgroup$ – anna v May 28 '19 at 6:42
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    $\begingroup$ @anna v I think we’re talking about elastic deformation not plastic (permanent) deformation, no? $\endgroup$ – Bob D May 28 '19 at 7:30
  • $\begingroup$ @BobD The title says "elastic collision", elastic deformation is a different story $\endgroup$ – anna v May 28 '19 at 7:36
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    $\begingroup$ @annav I think the deformation referred to in the question is transient rather than permanent. $\endgroup$ – dmckee --- ex-moderator kitten May 28 '19 at 7:47
  • $\begingroup$ @dmckee that is fine , except that the title does not say that. When spins and deformations enter it is not the cms elastic scatter that only changes angles. $\endgroup$ – anna v May 28 '19 at 7:49

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