\(\begin{array}{l}
= 1 - {\left( {\frac{{{m_2} - {m_1}}}{{{m_2} + {m_1}}}} \right)^2} = 1 - {\left( {\frac{{2 - 1}}{{2 + 1}}} \right)^2}\\
= 1 - {\left( {\frac{1}{3}} \right)^2} = 1 - \frac{1}{9} = \frac{8}{9}
\end{array}\)
Percentage loss in energy
\( = \frac{8}{9} \times 100 = 90\% \)