Solution 16.2
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(New page: Using <math>{{m}_{A}}{{v}_{A}}+{{m}_{B}}{{v}_{B}}={{m}_{A}}{{u}_{A}}+{{m}_{B}}{{u}_{B}}</math> and <math>{{v}_{A}}={{v}_{B}}=0</math> <math>\begin{align} & 0=40\times 3+2v \\ & v=-...)
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Using
\displaystyle {{m}_{A}}{{v}_{A}}+{{m}_{B}}{{v}_{B}}={{m}_{A}}{{u}_{A}}+{{m}_{B}}{{u}_{B}}
and
\displaystyle {{v}_{A}}={{v}_{B}}=0
\displaystyle \begin{align} & 0=40\times 3+2v \\ & v=-\frac{120}{2}=-60\text{ m}{{\text{s}}^{\text{-1}}} \\ \end{align}
Skate board moves in the opposite direction to the child at \displaystyle \text{60 }\ \text{m}{{\text{s}}^{\text{-1}}}.
