Answer
The mass of the other body is $~~1.2~kg$
Work Step by Step
The mass of the moving body is $m_1 = 2.0~kg$
Note that $~~v_{1f} = \frac{1}{4}~v_{1i}$
We can use Equation (9-67) to find $m_2$, the mass of the other body:
$v_{1f} = \frac{m_1-m_2}{m_1+m_2}~v_{1i}$
$\frac{1}{4}~v_{1i} = \frac{m_1-m_2}{m_1+m_2}~v_{1i}$
$m_1+m_2 = 4~(m_1-m_2)$
$5~m_2 = 3~m_1$
$m_2 = \frac{3}{5}~m_1$
$m_2 = (\frac{3}{5})~(2.0~kg)$
$m_2 = 1.2~kg$
The mass of the other body is $~~1.2~kg$
