## Essential University Physics: Volume 1 (4th Edition)

The slower one is now moving $11\ Mm/s$ while the faster one is now moving $6.9 \ Mm/s$.
We know the following equation for elastic collisions: $v_{1f}=\frac{m_1-m_2}{m_1+m_2}v_{1i}+\frac{2m_2}{m_1+m_2}v_{2i}$ Since their masses are equal, it follows: $v_{1f}=v_{2i}$ Thus, their velocities flip, so the slower one is now moving $11\ Mm/s$ while the faster one is now moving $6.9 \ Mm/s$. (They are moving in opposite directions.)