Time to fall double distance $a' = 0$ $v' = a$, $v_0 = 0$, so $v(t) = at$ $d' = v$, $d_0 = 0$, so $d(t) = \frac{at^2}{2}$ rearrange $t$ to subject: $t(d) = \sqrt\frac{2d}{a}$ Time for double distance is $t(2d) = \sqrt\frac{2(2d)}{a} = \sqrt{2}\sqrt\frac{2d}{a} = \sqrt{2}\cdot t(d)$