Like Vector Quantities are Multiples of Each Other
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Theorem
Let $\mathbf a$ and $\mathbf b$ be like vector quantities.
Then:
- $\mathbf a = \dfrac {\size {\mathbf a} } {\size {\mathbf b} } \mathbf b$
where:
- $\size {\mathbf a}$ denotes the magnitude of $\mathbf a$
- $\dfrac {\size {\mathbf a} } {\size {\mathbf b} } \mathbf b$ denotes the scalar product of $\mathbf b$ by $\dfrac {\size {\mathbf a} } {\size {\mathbf b} }$.
Proof
By the definition of like vector quantities:
- $\mathbf a$ and $\mathbf b$ are like vector quantities if and only if they have the same direction.
By definition of unit vector:
- $\dfrac {\mathbf a} {\size {\mathbf a} } = \dfrac {\mathbf b} {\size {\mathbf b} }$
as both are in the same direction, and both have length $1$.
By definition of scalar division:
- $\dfrac 1 {\size {\mathbf a} } \mathbf a = \dfrac 1 {\size {\mathbf b} } \mathbf b$
Hence, multiplying by $\size {\mathbf a}$:
- $\mathbf a = \dfrac {\size {\mathbf a} } {\size {\mathbf b} } \mathbf b$
$\blacksquare$
Sources
- 1921: C.E. Weatherburn: Elementary Vector Analysis ... (previous) ... (next): Chapter $\text I$. Addition and Subtraction of Vectors. Centroids: Addition and Subtraction of Vectors: $5$. Multiplication by a number