## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

We can use conservation of momentum to find the mass of the water $m_w$. Let $m_c$ be the mass of the train car. $p_f=p_0$ $m_f~v_f = m_c~v_0$ $(m_c+m_w)~v_f = m_c~v_0$ $m_w~v_f = m_c~(v_0-v_f)$ $m_w = \frac{m_c~(v_0-v_f)}{v_f}$ $m_w = \frac{5000~kg~(22.0~m/s-20.0~m/s)}{20.0~m/s}$ $m_w = 500~kg$ The mass of the water is 500 kg.