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A moving body of mass $m$ and velocity $3\,\text{km h}^{-1}$ collides with a body at rest of mass $2m$ and then sticks to it. The combined velocity will be:

1. $4\,\text{km h}^{-1}$
2. $3\,\text{km h}^{-1}$
3. $2\,\text{km h}^{-1}$
4. $1\,\text{km h}^{-1}$

Correct answer: $1\,\text{km h}^{-1}$

Solution

In a perfectly inelastic collision, momentum is conserved. Initial momentum is $m\times 3=3m$ (in km h$^{-1}$ units), and final mass is $m+2m=3m$. So final velocity is $3m/3m=1\,\text{km h}^{-1}$.

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