Answer
The mass of the body that was moving faster before the collision is equal to that of the other body.
Work Step by Step
Line segment 1 and line segment 3 correspond to the positions of the two bodies before the collision. Line segment 2 corresponds to the motion of the center of mass before the collision.
Since the motion of the center of mass does not change during a collision, line segment 5 corresponds to the motion of the center of mass after the collision.
Let block 2 be the block that was initially stationary. Line segment 4 corresponds to the motion of block 2 after the collision.
Let block 1 be the block that was moving before the collision. Line segment 6 corresponds to the motion of block 1 after the collision. Since line segment 6 is horizontal, we can see that block 1 is stationary after the collision.
We can use equation (9-67) to find an expression for the final velocity $v_{1f}$ of block 1 after the collision:
$v_{1f} = \frac{m_1-m_2}{m_1+m_2}~v_{1i}$
Since $v_{1f} = 0,$ then $m_1 = m_2$
The mass of the body that was moving faster before the collision is equal to that of the other body.
