Vector Algebra Question 24
Question: If the moduli of $\vec{a}$ and $\vec{b}$ are equal and angle between them is $120^{\circ}$ and $\vec{a} \cdot \vec{b}=-8$, then $| \vec{a} |$ is equal to
[RPET 1986]
Options:
A) - 5
B) - 4
C) 4
D) 5
Show Answer
Answer:
Correct Answer: C
Solution:
- $ \mathbf{a} .\mathbf{b}=aa\cos 120{}^\circ $ , $ { \because |\mathbf{a}| = |\mathbf{b}| =a (say) } $
$ \Rightarrow -8=-\frac{a^{2}}{2}\Rightarrow a=4 $ (Negative sign does not occur in module).
BETA
