5 pounds is a weight people can probably relate to, perhaps as a dumbbell weight. Or half gallon + 1.5 cup + 4 teaspoon of water. Or 5 pint - 6 tablespoon - 2 teaspoon ("pint is a pound").
We calculate the diameters of spheres of various materials that weigh 5 lb.
water 1 g/cc 6.4176889 inch
glass (fused quartz) 2.2 g/cc 4.9344388 inch
aluminum 2.70 g/cc 4.6088305 inch
iron 7.874 g/cc 3.2258699 inch
lead 11.35 g/cc 2.8557052 inch
depleted uranium 19.05 g/cc 2.4029625 inch
gold 19.32 g/cc 2.391716 inch
tungsten 19.35 g/cc 2.3904794 inch
iridium 22.4 g/cc 2.2766485 inch
Example input to 'units' program for iridium: (5 lb / (22.4 g / cc) / (4|3) / pi) ^ (1|3) * 2
Bogusly too many significant figures, of course. Round appropriately, probably based on the purity of your material and what your machining tools can do.
Subjectively, linear dimension does not decrease very much after lead because of the cube root.
Metric version:
2 kg is a weight people can probably relate to, e.g., 2 L of water. We calculate the diameters of spheres of various materials that weigh 2 kg.
water 1 g/cc 15.631853 cm
glass (fused quartz) 2.2 g/cc 12.019034 cm
aluminum 2.70 g/cc 11.225935 cm
iron 7.874 g/cc 7.8573962 cm
lead 11.35 g/cc 6.9557693 cm
depleted uranium 19.05 g/cc 5.8530037 cm
gold 19.32 g/cc 5.8256101 cm
tungsten 19.35 g/cc 5.8225979 cm
iridium 22.4 g/cc 5.545335 cm
Black holes vary immensely in average density. The more massive they are, the (counterintuitively) less dense they are. Large supermassive black holes have average density less than water. A black hole with mass 2 kg has Schwarzschild diameter 5.9e-27 m. Its density is 2 kg / (4|3 pi (2 kg 2 gauss_k^2 au^3 day^-2 c^-2 / solarmass)^3) = 1.8e76 g/cc.
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