Question

The magnitude of the resultant of two equal vectors is equal to the magnitude of either vector. What is the angle betwee the two vectors?

• A. ${60}^{\circ }$
• B. ${90}^{\circ }$
• C. ${120}^{\circ }$
• D. ${150}^{\circ }$

Answer 1

The correct option is C

${120}^{\circ }$

We know that, $R^2=A^2+B^2+2ABcos\theta$
It is given that R = A = B.
Putting these values we have
$A^2=A^2+A^2+2A^{2}cos\theta$
$\Rightarrow cos\theta =\frac{-1}{2}$ which gives $\theta ={120}^{\circ }$
Hence, the correct choice is (c).