Question:
A body of mass $2 \mathrm{~kg}$ makes an elastic collision with a second body at rest and continues to move in the original direction but with one fourth of its original speed. What is the mass of the second body?
Correct Option: , 4
Solution:
(4) For head on elastic collision we have
$\mathrm{V}_{1}=\frac{\left(\mathrm{m}_{1}-\mathrm{m}_{2}\right) \mathrm{u}_{1}}{\mathrm{~m}_{1}+\mathrm{m}_{2}}+\frac{2 \mathrm{~m}_{2} \mathrm{u}_{2}}{\mathrm{~m}_{1}+\mathrm{m}_{2}}$
Here $m_{1}=2 k g, u_{1}=x, u_{2}=0$,
$\mathrm{v}_{1}=\mathrm{x} / 4$
$\therefore \frac{x}{4}=\frac{\left(2-m_{2}\right) x}{2+m_{2}} \Rightarrow m_{2}=1.2 k g$
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