Answer: Their final relative velocity is -0.412 m/s.
Explanation:
According to the law of conservation,
   [tex]m_{1}v_{1} + m_{2}v_{2} = (m_{1} + m_{2})v[/tex]
Putting the given values into the above formula as follows.
   [tex]m_{1}v_{1} + m_{2}v_{2} = (m_{1} + m_{2})v[/tex]
   [tex]2.50 \times 10^{3} kg \times 0 m/s + 7.50 \times 10^{3} kg \times -0.550 m/s = (2.50 \times 10^{3} kg + 7.50 \times 10^{3} kg)v[/tex]
      [tex]-4.12 \times 10^{3} kg m/s = (10^{4} kg) v[/tex]
          v = [tex]\frac{-4.12 \times 10^{3} kg m/s}{10^{4} kg}[/tex]
           = -0.412 m/s
Thus, we can conclude that their final relative velocity is -0.412 m/s.
