The gravitational potential at a point above the surface of earth is −5.12×107 J/kg and the acceleration due to gravity at that point is 6.4 m/s2 . Assume that the mean radius of earth to be 6400 km. The height of this point above the earth's surface is :
(1) 1600 km
(2) 540 km
(3) 1200 km
(4) 1000 km