Two point masses connected by an ideal string are placed on a smooth horizontal surface as shown in the diagram. A sharp impulse of $10\mathrm{ }\mathrm{k}\mathrm{g }\mathrm{m} {\mathrm{s}}^{-1}$ is given to the $5\mathrm{ }\mathrm{k}\mathrm{g}$ mass at an angle of $60°$ to the line joining the masses. The velocity of the $10\mathrm{ }\mathrm{k}\mathrm{g}$ mass just after the impulse will be 
 

• A

  $\frac{2}{3}\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$
• Correct answer
  B

  $\frac{1}{3}\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$
• C

  $2\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$
• D

  zero

Solution

Apply impulse equation, along string



Impulse = $\mathrm{\Delta }p$
$10\mathrm{ }\times \mathrm{ }\frac{1}{2}=\left(10\mathrm{ }+\mathrm{ }5\right)\mathrm{v}$

$\mathrm{v}=\frac{1}{3}\mathrm{m} {\mathrm{s}}^{-1}$

Hence, the correct answer is option: 'B'.