If the radius of the earth suddenly decreases to 80% of its present value, the mass of the earth remaining the same, the value of the acceleration due to gravity will

Now, g=GM/R2If R reduces to R' = 0.8 R, the value of g becomes         g′=GMR2=GM0.64R2=g0.64=9.8ms−20.64which is choice (2). Increase in the value of g, =g-g0.64=0.36g0.64Therefore, the percentage increase =0.36g0.64g×100=56.25%Hence, choices (2) and (4) are correct.