Đáp án:

 câu 21

a.$45\left( m \right)$

b.$\left\{ \begin{array}{l}
{h_B} = 15\left( m \right)\\
{v_B} = 5\sqrt 6 \left( {m/s} \right)
\end{array} \right.$

câu 22

a. 50m

b.30m;20m/s

Giải thích các bước giải:

$\begin{array}{l}
21.\\
a.{{\rm{W}}_0} = {{\rm{W}}_A}\\
 \Rightarrow mg{h_0} + \frac{1}{2}mv_0^2 = mg{h_A}\\
 \Rightarrow 10.25 + \frac{1}{2}{.20^2} = 10.{h_A}\\
 \Rightarrow {h_A} = 45\left( m \right)\\
b.{{\rm{W}}_{tB}} = \frac{1}{2}{{\rm{W}}_{dB}}\\
 \Rightarrow \left\{ \begin{array}{l}
{{\rm{W}}_{tB}} = \frac{{\rm{W}}}{3}\\
{{\rm{W}}_{dB}} = \frac{{\rm{W}}}{6}
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
mg{h_B} = \frac{{mg{h_A}}}{3}\\
\frac{1}{2}mv_B^2 = \frac{{mg{h_A}}}{6}
\end{array} \right. \Rightarrow \left\{ \begin{array}{l}
{h_B} = 15\left( m \right)\\
{v_B} = 5\sqrt 6 \left( {m/s} \right)
\end{array} \right.\\
22.\\
{\rm{a}}{\rm{.}}{{\rm{W}}_{t1}} - {{\rm{W}}_{t2}} = mg\left( {{z_1} - {z_2}} \right)\\
 \Rightarrow 600 + 900 = 3.10.h\\
 \Rightarrow h = 50\left( m \right)\\
b.{{\rm{W}}_{t3}} - {{\rm{W}}_{t2}} = mg{h_0}\\
 \Rightarrow 900 = 3.10.{h_0}\\
 \Rightarrow {h_0} = 30\left( m \right)\\
{{\rm{W}}_{d3}} = {\rm{W}} = {{\rm{W}}_{t1}}\\
 \Rightarrow \frac{1}{2}mv_3^2 = {{\rm{W}}_{t1}}\\
 \Rightarrow \frac{1}{2}.3.v_3^2 = 600\\
 \Rightarrow {v_3} = 20\left( {m/s} \right)
\end{array}$