Answer
(a) The tension in coupling B decreases; the others stay the same.
(b) The tension in coupling C increases; the others stay the same.
Work Step by Step
Let's call the mass of car 1 $m_1$, the mass of car 2 $m_2$ and that of car 3 $m_3$ (including the luggage)
Because all cars move at the same acceleration $a=0.12m/s^2$, we have
- The tension in coupling bar A (pulls all 3 cars): $T_a=(m_1+m_2+m_3)a$
- The tension in coupling bar B (pulls car 2 and 3): $T_b=(m_2+m_3)a$
- The tension in coupling bar C (pulls car 3): $T_c=m_3a$
(a) If $39kg$ of luggage were moved from car 2 to car 1,
- The total mass $m_1+m_2+m_3$ of 3 cars does not change, so $T_a$ does not change.
- The mass $m_3$ does not change, so $T_c$ does not change.
- $(m_2+m_3)$ decreases, so $T_b$ decreases.
(b) If $39kg$ of luggage were moved from car 2 to car 3,
- The total mass $m_1+m_2+m_3$ of 3 cars does not change, so $T_a$ does not change.
- $m_3$ increases, so $T_c$ increases.
- $m_2$ decreases by $39kg$ but $m_3$ gains by $39kg$, so $m_2+m_3$ stays the same and $T_b$ does not change.
