(1) Centre a gravity (CG) 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 anybody can be considered to be concentrated at its CG.

(4) The radius of gyration of anybody rotating about an axis is the length of the perpendicular dropped from the CG of the body to the axis.

Which one of the following pairs of statements is correct?

• (4) and (1)

• (1) and (2)

• (2) and (3)

• (3) and (4)

A black hole is an object whose gravitational field is so strong that even light cannot escape from it. To what approximate radius would earth (mass = 5.98 x 10²⁴ kg) have to be compressed to be a black hole?

• 10^-9 m

• 10^-6 m

• 10^-2 m

• 100 m

A body mass 1 kg is thrown upwards with velocity 20 ms^-1. It momentarily comes to rest after attaining a height of 18 m. How much energy is lost due to air friction?  (g = 10 ms^-2)

• 20 J

• 30 J

• 40 J

• 10 J

A body projected vertically from the earth reaches a height equal to earth's radius before returning to the earth. The power exerted by the gravitational force is greatest

• at the instant just before the body hits the earth

• it remains constant all through

• at the instant just after the body is projected

• at the highest position of the body

A boy standing at the t op of a tower of 20 m height drops a stone. Assuming g = 10 ms^-2, the velocity with which it hits the ground is 

• 20 m/s

• 40 m/s

• 5 m/s

• 10 m/s