Question:

If a\vec{a} and b\vec{b} are two collinear vectors,then which of the following are incorrect:

Updated On: Feb 29, 2024
  • b=λa\vec{b}=λ\vec{a},for some scalar λλ

  • a=±b\vec{a}=±\vec{b}

  • the respective components of a\vec{a} and b\vec{b} are proportional

  • both the vectors a\vec{a} and b\vec{b} have same direction,but different magnitudes

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The Correct Option is D

Solution and Explanation

The correct answer is D:both the vectors a\vec{a} and b\vec{b} have same direction,but different magnitudes
If a\vec{a} and b\vec{b} are two collinear vectors,then they are parallel.
Therefore, we have:
b=λa\vec{b}=λ\vec{a}(for some scalar λλ)
If λ=±1λ=±1,then a=±b.\vec{a}=±\vec{b}.
If a=a1i^+a2j^+a3k^\vec{a}=a_1\hat{i}+a_2\hat{j}+a_3\hat{k} and b=b1i^+b2j^+b3k^\vec{b}=b_1\hat{i}+b_2\hat{j}+b_3\hat{k},then
b=λa.\vec{b}=λ\vec{a}.
    b1i^+b2j^+b3k^\implies b_1\hat{i}+b_2\hat{j}+b_3\hat{k}=λ(a1i^+a2j^+a3k^)=\lambda(a_1\hat{i}+a_2\hat{j}+a_3\hat{k})
    b1i^+b2j^+b3k^\implies b_1\hat{i}+b_2\hat{j}+b_3\hat{k}=(λa1)i^+(λa2)j^+(λa3)k^=(λa_1)\hat{i}+(λa_2)\hat{j}+(λa_3)\hat{k}
b1=λa1,b2=λa2,b3=λa3⇒b_1=λa_1,b_2=λa_2,b_3=λa_3
b1a1=b2a2=b3a3=λ⇒\frac{b_1}{a_1}=\frac{b_2}{a_2}=\frac{b_3}{a_3}=λ
Therefore,the respective components of a\vec{a} and b\vec{b} are proportional.
However,vectors a\vec{a} and b\vec{b} can have different directions.
Hence,the statement given in D is incorrect.
The correct answer is D.
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