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Well, the question says it all; is there a definition of body in physics? What is to be considered a physical object and what it cannot?

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See en.wikipedia.org/wiki/Body_(physics) - In mechanics, it's just a collection of particles for which it's useful to treat them as a whole. - Clearly, such semi-informal terms aren't supposed to have a super-rigorous definition. Quite generally, in physics, we allow the exact meaning of such words to be evolved as we switch from more primitive theories to more sophisticated ones. As we're changing our model of the world, we're changing the opinion about the character of objects. There are words corresponding to well-defined quantities with a number etc. but "body" isn't one of them. –  Luboš Motl Apr 28 '13 at 9:22
It's bizarre that you added the URL to Wikipedia a minute after I wrote the comment above - which included the very same URL. This makes my comment look a bit silly so I need to point out that your updated, "improved" question that takes some of the comment into account is a form of mild plagiarism. Authors of questions shouldn't be updating their questions in ways that try to make it look like they knew something while asking the question in the first place that they actually didn't! It's really fraudulent and misses the point of why the questions are (or should be) being asked and answered. –  Luboš Motl Apr 28 '13 at 9:24
@LubošMotl: Ups, the edit was mine, not OP's. I only saw your comment afterwards, to which I fully agree. Should this question be closed as, say, not constructive? –  Qmechanic Apr 28 '13 at 9:50
@Luboš your first comment should probably be an answer, and your second comment isn't true. As a question author learns new information, it's acceptable and encouraged for them to either edit it into the question, or post it as an answer if it constitutes an answer. If it makes your comment look silly, just delete the comment. (Although it wasn't the OP's edit in this case) –  David Z Apr 28 '13 at 9:52
That being said, Matteo, did you look at the linked Wikipedia page before posting here? If so, what about it didn't you understand? If you didn't, you should take a look at it now and see whether it answers your question. It's expected that you do some basic research (such as checking Wikipedia and a web search) before posting a question here. –  David Z Apr 28 '13 at 9:55
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