A block of mass $10~\text{kg}$ moving with a speed of $5~\text{m/s}$ on a frictionless horizontal surface suddenly explodes into two pieces.
If one piece with mass $4~\text{kg}$ moves with a speed of $10~\text{m/s}$, then the velocity of the second piece is
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Use momentum conservation when no external force acts. Total momentum before = after.