Đáp án:

 \({m_2} = \dfrac{1}{9}kg\)

Giải thích các bước giải:

Ta có: 

\(\begin{array}{l}
{W_1} = \dfrac{1}{2}{m_1}v_1^2 = \dfrac{1}{2}{.1.6^2} = 18J\\
{W_2} = 0,9.{W_1} = 0,9.18 = 16,2J\\
{m_1}{v_1} = \left( {{m_1} + {m_2}} \right){v_2}\\
 \Rightarrow {v_2} = \dfrac{{{m_1}}}{{{m_1} + {m_2}}}{v_1} = \dfrac{6}{{1 + {m_2}}}\\
{W_2} = \dfrac{1}{2}\left( {{m_1} + {m_2}} \right)v_2^2\\
 \Rightarrow 16,2 = \dfrac{1}{2}\left( {1 + {m_2}} \right).{\left( {\dfrac{6}{{1 + {m_2}}}} \right)^2}\\
 \Rightarrow {m_2} = \dfrac{1}{9}kg
\end{array}\)