Solution to 1996 Problem 20

Recall the following formula for a 1D elastic collision in which particle 2 is initially at rest:
\begin{align*}v' = \frac{m_1 - m_2}{m_1 + m_2} v\end{align*}
where v and v' are the initial and final velocities of the particle initially not at rest. Plug in m_1 = 4 and v' = -0.6v.
\begin{align*}-0.6 v = \frac{4 u - m_2}{4 u + m_2} v\end{align*}
Then solve for m_2:
\begin{align*}0.4 m_2 = 4 u + 2.4 u = 6.4 u  \Rightarrow m_2 = \boxed{16 u}\end{align*}
Therefore, answer (D) is correct.

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