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Particles each of mass m and velocity v are fired from a source at a rate of n per second along a straight line. They strike a body of mass M (initially at rest) and stick to it. What is the speed of the body after it has absorbed N particles?

1. mvn/(Nm+n)
2. mvN/(Nm+M)
3. mv/(Nm+M)
4. (Nm+M)/(mv)

Correct answer: mvN/(Nm+M)

Solution

After N particles (total momentum Nmv, total mass Nm) embed in M, momentum conservation gives speed = Nmv/(M + Nm).

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