Question

Two vectors of same magnitude have a resultant equal to either, then the angle between the vectors will be:

• A. $20{}^{\circ }$
• B. $135{}^{\circ }$
• C. $120{}^{\circ }$
• D. $45{}^{\circ }$

Answer 1

Answer: (C)

Given : $\vec{A}=\vec{B}=\vec{R}$ Resultant, $R^2=A^2+B^2+2AB\cos \theta$ $A^2=A^2+A^2+2AA\cos \theta$ $1=1+1+2\cos \theta$ $2\cos \theta =1-2=-1$ $\cos \theta =-\frac{1}{2}=\cos {120}^o$ $\theta ={120}^o$