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。
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。
The other way to think about it, if I had a flat object then the center of mass would basically be the point where I would need to hold it so it is perfectly balanced.
从另一个角度看，一个平面物体的重心就是,如果在重心处拿起物体，它就能完全地保持平衡。
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