The unit of length convenient on the nuclear scale is a fermi : 1 f = `10^(–15)` m. Nuclear sizes obey roughly the following empirical relation : `r=r_oA^(1/3)` where r is the radius of the nucleus, A its mass number, and `r_o` is a constant equal to about, 1.2 f. Show that the rule implies that nuclear mass density is nearly constant for different nuclei. Estimate the mass density of sodium nucleus. Compare it with the average mass density of a sodium atom obtained in Exercise. 2.27.