a,
$W_{t_o}= mgh_o= 0,1.10.45= 45(J)$
b,
$W_o= W_{t_o}= 45(J)$
$W_đ= W_{d_đ}= 45 (J)$ (bảo toàn cơ năng)
$\Rightarrow 0,5mv_đ^2= 45$
$\Leftrightarrow v_đ= 30(m/s)$
$P_đ= mv_đ= 0,1.30= 3 (kg.m/s)$
c,
$s= \frac{2}{3}.45= 30(m/s)$
$\Rightarrow h= 45-30=15(m)$
$W_t= mgh= 0,1.10.15= 15(J)$
$W_t + W_d= W= 45(J)$
$\Rightarrow W_d= 45-15=30(J)$
d,
Tại vị trí $W_d= \frac{1}{3}W= \frac{1}{3}W_d+ \frac{1}{3}W_t$
$\Leftrightarrow W_t= 2W_d$
$W= W_d+ W_t= 3W_d= 45$
$\Leftrightarrow W_d= 0,5mv^2=15$
$\Leftrightarrow v= 10\sqrt{3}(m/s)$
$W_t= 2W_d= 30 (J)= mgh$
$\Leftrightarrow h= 30(m)$
$v= gt \Leftrightarrow t= \frac{10\sqrt{3}}{10}= \sqrt{3} (s)$