Answer
(a) $D = 0.28~kg/m$
(b) $v_t = 42~m/s$
Work Step by Step
(a) $v_t = \sqrt{\frac{mg}{D}}$
$D = \frac{mg}{v_t^2} = \frac{(50~kg)(9.80~m/s^2)}{(42~m/s)^2}$
$D = 0.28~kg/m$
(b) $v_t = \sqrt{\frac{mg}{D}}= \sqrt{\frac{(45~kg)(9.80~m/s^2)}{0.25~kg/m}}$
$v_t = 42~m/s$
