Đáp án:

\(\begin{array}{l}
21.B.6,25m/s\\
{{\rm{W}}_{d\max }} = {W_{t\max }} \Rightarrow \dfrac{1}{2}{m_1}{v^2} = {m_1}gh\\
 \Rightarrow \frac{1}{2}.{v^2} = 10.5 \Rightarrow v = 10m/s\\
p = p'\\
 \Rightarrow {m_1}v = ({m_1} + {m_2})v' \Rightarrow 5.10 = (5 + 3)v'\\
 \Rightarrow v' = 6,25m/s\\
22.B.1200J,P = 60W\\
A = Ph = 10.15.8 = 1200J\\
P = \dfrac{A}{t} = \dfrac{{1200}}{{20}} = 60W\\
23.D.A = 1320J,P = 330W\\
s = \dfrac{1}{2}a{t^2} \Rightarrow a = \dfrac{{2s}}{{{t^2}}} = \dfrac{{2.8}}{{{4^2}}} = 1m/{s^2}\\
F - P = ma \Rightarrow F = ma + mg = 15.1 + 15.10 = 165N\\
A = Fs = 165.8 = 1320J\\
P = \dfrac{A}{t} = \dfrac{{1320}}{4} = 330W\\
24.B.A = 20,7J\\
{A_1} = P{h_1} = 1.10.1.\sin 60 = 5\sqrt 3 J\\
{A_2} = P{h_2} = 1.10.1.\sin 45 = 5\sqrt 2 J\\
{A_3} = P{h_3} = 1.10.1.\sin 30 = 5J\\
A = {A_1} + {A_2} + {A_3} = 5\sqrt 3  + 5\sqrt 2  + 5 = 20,7J\\
25.\\
h = \dfrac{1}{2}a{t^2} \Rightarrow a = \dfrac{{2s}}{{{t^2}}} = \dfrac{{2.10}}{{{5^2}}} = 0,8m/{s^2}\\
F - P = ma \Rightarrow F = ma + mg = 5000.0,8 + 5000.10 = 54000N\\
{s_4} = \dfrac{1}{2}at_4^2 = \dfrac{1}{2}.0,{8.4^2} = 6,4m\\
s = h - {s_4} = 10 - 6,4 = 3,6m\\
A = Fs = 54000.3,6 = 194400J
\end{array}\)