No one wants to deal with that many interactions. Instead, in physics we treat the ball as one thing—and that’s mostly fine. However, to make sure that other people understand what we are doing when we model an interaction, we have to define our “system.” Perhaps, to make it easy, we decide the system is just the ball itself. If so, we deal only with the ball’s momentum and any forces due to external interactions, and we can ignore all those atom-atom interactions. We could even forget about the interaction between the ball’s fuzzy surface and its interior rubber part.
It’s also possible to have a system that consists of
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