Question

on a planet (whose size is the same and mass 4 times as that of the earth) the energy needed to lift a 2kg mass vertically upwards through 2m distance on the planet is

Answer 1

The acceleration due to gravity is given by g=GMR2. So in the other planet the acceleration due to gravity is given by 
g'=G4MR2=4GMR2=4g
So energy to lift 2 kg mass to 2 m height is given by 
Ep=mg'h=2×4g×2=2×4×10×2=160 J

Answer 2

Answer:

Here , RP = Re;MP = 4Me

M = 3kg ; h = 3m ; ge =10m/s^−2

On the surface of the earth,

ge = GM/Re^2

On the surface of the planet,

gP = GMp/Rp^2 = (G4Me/Re^2)4(GMe/Re^2 = 4ge

gP = 4 × 10 = 40m/s^2

Work done = mgPh = 3 × 40 × 3 = 360J.