Gravitation

At a certain depth "d " below surface of earth, value of acceleration due to gravity becomes four times that of its value at a height \(\mathrm{3 R}\) above earth surface. Where \(\mathrm{R}\) is Radius of earth (Take \(\mathrm{R}=6400 \mathrm{~km}\) ). The depth \(\mathrm{d}\) is equal to