The unit of length convenient on nuclear scale is a fermi, `1f = 10^9-15)`m. Nuclear sizes obey rougholy the following empricial relation : `r = r_0 A^(1//3)`, where r is radius of the nucleus and `r_0` is a constant equal to 1.2 f. show that the rule implies that nuclear mass density in nearly constant for different neclei. Estimate the mass density of sodium nucleus. Compare it with avarge mass density of sodium atom is Q. 27 `(4.67xx10^3 kg//m^3)`.