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.,

$F = G \frac{m_{1} m_{2}}{r^{2}}$

If distance $r$ becomes $r/2$ , then the gravitational force will be proportional to

$\therefore F ^{\prime}= G \frac{m_{1} m_{2}}{\left(\frac{r}{2}\right)^{2}}$

$\therefore F ^{\prime}=4 G \frac{m_{1} m_{2}}{r^{2}}$

$\therefore F ^{\prime}=4 F$

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