A body of mass


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?

  1. (1) $1.0 \mathrm{~kg}$

  2. (2) $1.5 \mathrm{~kg}$

  3. (3) $1.8 \mathrm{~kg}$

  4. (4) $1.2 \mathrm{~kg}$

Correct Option: , 4


(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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