a hollow metal sphere has an internal radius of 20cm and an external radius of 30cm , given that the density of the metal is 7.8, find the mass of the sphere , expressing your answer in kg.

Answer 1

just multiply the volume of metal by its density to get the mass:

cm^2 * g/cm^3 = g

You know that the volume of a sphere of radius r is

v = 4/3 πr^3

so, to get the volume of metal, just subtract the volume of the empty inner sphere from the outer sphere.

Answer 2

the volume of metal is
... v = 4/3 * π * (30³ - 20³) cm³

if the density units are g/cm³
then multiply v by .0078