Answer
The railroad car's speed just after the gravel is loaded is 1.4 m/s
Work Step by Step
We can use conservation of momentum to find the final speed of the railroad car after the gravel is loaded.
$p_f=p_0$
$m_f~v_f = m_0~v_0$
$v_f = \frac{m_0~v_0}{m_f}$
$v_f = \frac{(10,000~kg)(2.0~m/s)}{10,000~kg+4000~kg}$
$v_f = 1.4~m/s$
The railroad car's speed just after the gravel is loaded is 1.4 m/s.
