Add grams of mass to one of the cars.

When released, it should take seconds to drop.

It should hit the floor with a speed of m/s.

\begin{array}{l} m_{{\rm{full}}} g - m_{{\rm{empty}}} g = \left( {m_{{\rm{full}}} + m_{empty} } \right)a \\ m_{{\rm{full}}} g - m_{{\rm{empty}}} g = am_{{\rm{full}}} + am_{empty} \\ m_{{\rm{full}}} g - am_{{\rm{full}}} = am_{empty} + m_{{\rm{empty}}} g \\ m_{{\rm{full}}} \left( {g - a} \right) = m_{empty} \left( {a + g} \right) \\ m_{{\rm{full}}} = \frac{{m_{empty} \left( {a + g} \right)}}{{\left( {g - a} \right)}} \\ m_{{\rm{add}}} = m_{{\rm{full}}} - m_{{\rm{empty}}} \\ \\ t = \sqrt {\frac{{2h}}{a}} \\ v = \sqrt {2ha} \\ \end{array}