A particle of mass m is thrown upwards from the surface of the earth, with a velocity u. The mass and the radius of the earth are, respectively, M and R. G is gravitational constant and g is acceleration due to gravity on the surface of the earth. The minimum value of u so that the particle does not return back to earth, is

A particle of mass M is situated at the centre of a spherical shell of same mass and radius a. The magnitude of the gravitational potential at a point sutuated at a/2 distance from the centre, will be :

A planet moving along an elliptical orbit is closest to the sun at a distance r₁ and farthest away at a distance of r₂. If $$v$$₁ and $$v$$₂ are the linear velocities at these points respectively, then the ratio $${{{v_1}} \over {{v_2}}}$$ is

(1)   Centre of gravity (C.G) of a body is the point at which the weight of the body acts
(2)   Centre of mass coincides with the centre of gravity if the earth is assumed to have infinitely large radius.
(3)   To evaluate the gravitational field intensity due to any body at an external point, the entire mass of the body can be considered to be concentrated at its C.G.
(4)   The radius of gyration of any body rotating about an axis is the length of the perpendicular dropped from the C.G. of the body to the axis.

Which one of the following pairs of statements is correct ?