Đáp án:

\(\begin{array}{l}
4.\\
F = 4,5N\\
5.\\
v = 0,9565m/s\\
6.\\
a.\\
v = 3,4m/s\\
b.\\
v = 0,2m/s
\end{array}\)

Giải thích các bước giải:

\(\begin{array}{l}
4.\\
v = \sqrt {2gh}  = \sqrt {2.10.3}  = \sqrt {60} m/s\\
\Delta p = p' - p = m(v' - v) = 60(0 - \sqrt {60} ) =  - 464,758kg.m/s\\
F = |\frac{{\Delta p}}{{\Delta t}}| = |\frac{{ - 464,758}}{{0,55}}| = 4,5N\\
5.\\
p = p'\\
 - {m_1}{v_1} + {m_2}{v_2} = ({m_1} + {m_2})v\\
v = \frac{{{m_1}{v_1} - {m_2}{v_2}}}{{{m_1} + {m_2}}} = \frac{{ - 0,3.2 + 2.0,8}}{{0,3 + 2}} = 0,9565m/s\\
6.\\
{p_1} = {m_1}{v_1} = 60.4 = 240kgm/s\\
{p_2} = {m_2}{v_2} = 90.3 = 270khm/s\\
a.\\
p = p'\\
{p_1} + {p_2} = ({m_1} + {m_2})v\\
v = \frac{{{p_1} + {p_2}}}{{{m_1} + {m_2}}} = \frac{{240 + 270}}{{60 + 90}} = 3,4m/s\\
b.\\
p = p'\\
 - {p_1} + {p_2} = ({m_1} + {m_2})v\\
v = \frac{{ - {p_1} + {p_2}}}{{{m_1} + {m_2}}} = \frac{{ - 240 + 270}}{{60 + 90}} = 0,2m/s
\end{array}\)