Astronomers know that Pluto and Triton have nearly the same size, mass, and density.

天文学家们知道冥王星和海王卫一拥有几乎相同的体积、质量和密度。

《柯林斯英汉双解大词典》

So, you can get the mass of a flat object with density.

因此，可以通过密度求一个平面物体的质量。

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The density for a flat object would be the mass per unit area.

一个平面物体的密度是每单位面积元的质量。

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The density for a flat object would be the mass per unit area.

一个平面物体的密度是每单位面积元的质量。

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