The distance through which one has to dig the Earth from its surface, so as to reach the point where the acceleration due to gravity is reduced by 40% of that at the surface of the Earth, is (radius of Earth is 6400 km)

Infinite number of masses each of 3kg are placed along a straight line at the distances of 1 m, 2m, 4m, 8m, ...... from a point O on the same line. If G is the universal gravitational constant, then the magnitude of gravitational field intensity at O is

A particle is kept on the surface of a uniform sphere of mass 1000 kg and radius 1 m. The work done per unit mass against the gravitational force between them is

[G = 6.67 $$\times$$ 10$$^{-11}$$ Nm$$^2$$ kg$$^{-2}$$]