来自《耶鲁大学开放课程：基础物理 第7集 第7集》
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You cannot see it but you know it's there.你看不到它 但你知道它的存在 -
How do you know it's there?你怎么就知道它存在呢 -
You know it's there我们知道它的存在 -
because our galaxy or any galaxy是因为我们银河系 或其他任何星系 -
has got matter you see and matter you don't see.都有你看得到和看不到的物质 -
If you draw an orbit around the galaxy,如果你画下一条这个星系的轨道 -
going all the way around the galaxy,绕着这个星系运行 -
you can treat the entire mass as sitting at the center.可以认为全部的质量都集中在中心 -
Then you can use your gravitational formula,你可以运用万有引力公式 -
v^2·r as the enclosed mass.v^2·r 就等于质量 -
If you go further out,如果向外扩张一点 -
you should find you're not enclosing any more mass你发现包含的质量没有随着增加