a,
$W_t= mgz= 0,5.10.30= 150J$
b,
Tại mặt đất, $z=0 \rightarrow W_t= 0J$
$v= \sqrt{2gh}= \sqrt{2.10.30}= 10.\sqrt{6} m/s$
=> $W_d= \frac{1}{2}mv^2= \frac{1}{2}.0,5.(10.\sqrt{6})^2= 150J$
c,
$W_d= 75J \rightarrow \frac{1}{2}mv^2= 75 \rightarrow v= 10.\sqrt{3} m/s$
$v^2= 2gh \rightarrow h= \frac{v^2}{2g}= 15m$
=> $z= 30-15= 15m$
=> $W_t= mgz= 75J$