[填空题]
1.State the law of conservation of momentum.
2.A stationary body explodes into two pieces of mass as $m_1$ and $m_2$ in opposite directions.
Just after the explosion, the ratio of the kinetic energy of $m_1$ to the kinetic energy of $m_2$ is found to be 9. Calculate the ratio of masses $\frac{m_2}{m_1}$ =
