Question

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

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

Answer 1

The correct option is D

${120}^{\circ }$

We know that,

$R^2=A^2+B^2+2AB\cos \theta .......\left(1\right)$

Given: $R=A=B$

Putting these values in eqn. $\left(1\right)$, we get

$A^2=A^2+A^2+2A^2\cos \theta$

$\Rightarrow \cos \theta =-\frac{1}{2}$

$\therefore \theta ={120}^{\circ }$

Hence, the correct option is $\left(\text{C}\right)$.