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# A metal ball-bearing with a circumference of 43.1 mm weighs 12.1 g. What is the density of the metal in g/cm3?

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- Roger the MoleLv 74 months agoFavorite Answer
The formula for the circumference of a sphere is the same as the formula for the circumference of a circle, since a circle is what you get when you bisect a sphere through it's center: 2πr, where r is the radius.

Solve this equation (C = 2πr) for the radius in terms of the circumference:

r = C/2π

The formula for the volume of a sphere is (4/3)πr^3. Substitute the expression equal to r (found above) in order to find the volume in terms of the circumference:

V = (4/3)π(C/(2π))^3 = C^3/(6 π^2)

Then substitute the given value for the circumference to determine the volume:

(43.1 mm)^3) / (6 π^2) = 1352.01 mm^3 = 1.35201 cm^3

(12.1 g) / (1.35201 cm^3) = 8.95 g/cm^3

[You didn't ask, but the common metal with a density like that is copper.]

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