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Well, you might say, "Look, we already did this last week,
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when I studied the Solar System,
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where there are planets moving around the Sun
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so that makes at least two bodies, the Sun and the planet."
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But actually,
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the Sun was not doing anything interesting in our analysis.
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The Sun just stood there as a source of the gravitational force.
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It's the planet that did all the orbiting
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and that was the problem in two dimensions,
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but of only one body.
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So now, we are going to enlarge our domain
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to more than one body obeying Newton's Laws.