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Question: Answered & Verified by Expert
Two forces with equal magnitudes $F$ act on a body and the magnitude of the resultant force is $F / 3$. The angle between the two forces is
PhysicsLaws of MotionJEE Main
Options:
  • A $\cos ^{-1}\left(-\frac{17}{18}\right)$
  • B $\cos ^{-1}\left(-\frac{1}{3}\right)$
  • C $\cos ^{-1}\left(\frac{2}{3}\right)$
  • D $\cos ^{-1}\left(\frac{8}{9}\right)$
Solution:
2361 Upvotes Verified Answer
The correct answer is: $\cos ^{-1}\left(-\frac{17}{18}\right)$
$\begin{aligned}
& F_{m t}^2=F_1^2+F_2^2+2 F_1 F_2 \cos \theta \\
& \Rightarrow\left(\frac{F}{3}\right)^2=F^2+F^2+2 F^2 \cos \theta \Rightarrow \cos \theta=\left(-\frac{17}{18}\right)
\end{aligned}$

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