E aí, Ismael. Uma ideia aqui é usar conservação de energia.
[tex3]\text{Em}_{\text{A}} = \text{Em}_{\text{B}} \,\,\, \Rightarrow \,\,\, \frac {\text{k} \cdot \Delta\text{x}_{\text{A}}^2}{2} + \text{m} \cdot \text{g} \cdot \text{h}_{\text{A}} = \frac {\text{k} \cdot \Delta\text{x}_{\text{B}}^2}{2} + \text{m} \cdot \frac{\text{v}_{\text{A}}^2}{2} \,\,\, \Rightarrow \,\,\, \\\\
\frac {40 \cdot (0,1)^2}{2} + 0,8 \cdot 10 \cdot 0,4= \frac { 40 \cdot (0,3)^2}{2} + 0,8 \cdot \frac{\text{v}_{\text{A}}^2}{2} \,\,\,\, \therefore \,\,\,\, \text{v}_{\text{A}} = 2 \, \text{m/s}
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