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A shell of mass $m$ moving with velocity $u$ suddenly breaks into two pieces. The part having mass $m/4$ remains stationary. The velocity of the other piece will be:

1. u
2. 2u
3. 3u/4
4. 4u/3

Correct answer: 4u/3

Solution

Since no external force acts, momentum is conserved. The initial momentum is $mu$, and after breaking, only the mass $3m/4$ moves, so its speed must be $\frac{mu}{3m/4}=\frac{4u}{3}$.

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