Đáp án:

 \(6000\left( {kgm/s} \right)\)

Giải thích các bước giải:

Ta có: 

\(\begin{array}{l}
p = \left( {{m_1} + {m_2}} \right)v = \left( {15 + 5} \right).300 = 6000kgm/s\\
{p_1} = {m_1}{v_1} = 15.400\sqrt 3  = 6000\sqrt 3 kgm/s\\
{p_2} = \sqrt {{p^2} + p_1^2}  = \sqrt {{{6000}^2} + {{\left( {6000\sqrt 3 } \right)}^2}}  = 12000kgm/s\\
{v_2} = \dfrac{{{p_2}}}{{{m_2}}} = \dfrac{{12000}}{5} = 2400\left( {m/s} \right)\\
\cos \alpha  = \dfrac{p}{{{p_2}}} = \dfrac{{6000}}{{12000}} = \dfrac{1}{2}\\
{v_{02}} = {v_2}\cos \alpha  = 2400.\dfrac{1}{2} = 1200\left( {m/s} \right)\\
{p_{2\min }} = {m_2}{v_{02}} = 5.1200 = 6000\left( {kgm/s} \right)
\end{array}\)