alleryTwo non-zero vectors $\vec{a}$ and $\vec{b}$ are perpendicular if and only if:
A$\vec{a} \times \vec{b} = 0$
B$\vec{a} \cdot \vec{b} = 0$
C$\vec{a} + \vec{b} = 0$
D$|\vec{a}| = |\vec{b}|$
Answer & Solution
Correct answer: B. $\vec{a} \cdot \vec{b} = 0$
$\vec{a} \cdot \vec{b} = |\vec{a}||\vec{b}|\cos\theta$. For two non-zero vectors, this is zero exactly when $\cos\theta = 0$, i.e. $\theta = 90°$.
Option B describes *parallel* vectors (cross product zero). Option D would require the vectors to be exact opposites of each other, not just perpendicular.
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