Question

Given that $\vec{A}+\vec{B}=\vec{R}$ and $A=B=R$. What should be the angle between $\vec{A}$ and $\vec{B}$ ?

• A. 0
• B. $\pi /3$
• C. $2\pi /3$
• D. $\pi$

Answer 1

Answer: (C)

$R^2=A^2+B^2+2AB\cos \theta$
$R^2=R^2+R^2+2R^2\cos \theta$
$2\cos \theta =-1$
$\Rightarrow \cos \theta =-\frac{1}{2}$
$\Rightarrow \theta =\frac{2\pi }{3}$