Question

The resultant of two forces which are equal in magnitude is equal to either of two vectors in magnitude. Find the angle between the forces.

• A. ${60}^{\circ }$
• B. ${45}^{\circ }$
• C. ${90}^{\circ }$
• D. ${120}^{\circ }$

Answer 1

The correct option is D ${120}^{\circ }$

Let F be the magnitude of each of th vectors as well as its resultant.

Using parallelogram law,
$F=\sqrt{F^2+F^2+2F^2\cos \theta }$
$\therefore \cos \theta =\frac{-1}{2}$
$\therefore \theta ={120}^{\circ }$