According to the universal law of gravitation, gravitational force (F) acting between two objects is inversely proportional to the square of the distance (r) between them, i.e.,

f ∝ 1 / r ^{2 }

If distance r becomes r/^{2}, then the gravitational force will be proportional to

1 / ( r/2 ) ^{2} = 4 / r ^{2 }

Hence, if the distance is reduced to half, then the gravitational force becomes four times larger than the previous value.