Because the gravitational force of the spherical shell to the inside of the shell is zero, at a depth of 6.4KM, it can be seen that it is on a small earth with a radius of r=R-6.4=999/ 1000R. Then, according to Mg = GMM/R 2, it is noticed that the volume of V = 4 π r 3/3 will decrease due to the change of radius, and the density of M with constant density will also decrease, so M=Vρ is substituted into it.