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3-1.Vectors
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The resultant of two forces, one double the other in magnitude, is perpendicular to the smaller of the two forces. The angle between the two forces is ........ $^o$
A
$60$
B
$120$
C
$150$
D
$90$
Solution

(b) $\tan \alpha = \frac{{2F\sin \theta }}{{F + 2F\cos \theta }} = \infty $ (as $\theta = 90^°$)
$⇒$ $F + 2F\cos \theta = 0$
$⇒$ $\cos \theta = – \frac{1}{2}$
$\theta = 120^\circ $
Standard 11
Physics