Magnitude of Vector Quantity in terms of Components
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Theorem
Let $\mathbf r$ be a vector quantity embedded in a Cartesian $3$-space.
Let $\mathbf r$ be expressed in terms of its components:
- $\mathbf r = x \mathbf i + y \mathbf j + z \mathbf k$
where $\mathbf i$, $\mathbf j$ and $\mathbf k$ denote the unit vectors in the positive directions of the $x$-axis, $y$-axis and $z$-axis respectively.
Then:
- $\size {\mathbf r} = \sqrt {x^2 + y^2 + z^2}$
where $\size {\mathbf r}$ denotes the magnitude of $\mathbf r$.
Proof
Let the initial point of $\mathbf r$ be $\tuple {x_1, y_1, z_1}$.
Let the terminal point of $\mathbf r$ be $\tuple {x_2, y_2, z_2}$.
Thus, by definition of the components of $\mathbf r$, the magnitude of $\mathbf r$ equals the distance between $\tuple {x_1, y_1, z_1}$ and $\tuple {x_2, y_2, z_2}$.
The result follows from Distance Formula in 3 Dimensions.
$\blacksquare$
Sources
- 1921: C.E. Weatherburn: Elementary Vector Analysis ... (previous) ... (next): Chapter $\text I$. Addition and Subtraction of Vectors. Centroids: Components of a Vector: $7$. The unit vectors $\mathbf i$, $\mathbf j$, $\mathbf k$
- 1970: George Arfken: Mathematical Methods for Physicists (2nd ed.) ... (previous) ... (next): Chapter $1$ Vector Analysis $1.1$ Definitions, Elementary Approach: $(1.7 a)$