a hole on earth:

1)隻有一個地球,沒有其他影響.R=6400km.

石離地心x,以地心作無限個同xin球麵,僅小與X為半徑的球麵對石作用,可設質量在心,令石質量為m,球為M,

We have mg=kmM/R^2,   stone"s force is kmM(x/R)^3/x^2=mg(x/R)=-ma=-md^2x/d^2t.We have d^2x/d^2t=-(g/R)x, x(t)=Acos(wt) w=(g/R)^.5=0.00124sec^(-1).T=808sec=13.47min.

Please chech it.