Two forces of magnitude $P$ & $Q$ acting at a point have resultant $R$. The resolved  part of $R$ in the direction of $P$ is of magnitude $Q$. Angle between the forces is :

  • A

    $2{\sin ^{ - 1}}\left( {\frac{P}{{2Q}}} \right)$

  • B

    $2{\sin ^{ - 1}}\left( {\frac{P}{{2Q}}} \right)^{\frac{1}{2}}$

  • C

    $2{\cos ^{ - 1}}\left( {\frac{P}{{2Q}}} \right)$

  • D

    $2{\cos ^{ - 1}}\left( {\frac{P}{{2Q}}} \right)^{\frac{1}{2}}$

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  • [AIIMS 2009]

The resultant of two vectors $\overrightarrow P $ and $\overrightarrow Q $ is $\overrightarrow R .$ If $Q$ is doubled, the new resultant is perpendicular to $P$. Then $R $ equals