Figure 3.13 Unit vectors
$\hat{i}$,
$\hat{j}$,
and
$\hat{k}$,
define the directions of a right-handed coordinate system.

A unit vector is a vector that has a magnitude of exactly 1 and points in a particular
direction. It lacks both dimension and unit. Its sole purpose is to point - that is,
to specify a direction. The unit vectors in the positive directions of the x, y, and
z axes are labeled
$\hat{i}$,
$\hat{j}$,
and
$\hat{k}$,
where the hat is used instead of an overhead arrow as for other vectors (Fig. 3-13).
The arrangement of axes in Fig. 3-13 is said to be a *right-handed coordinate system*.
The system remains right-handed if it is rotated rigidly. We use such coordinate
systems exclusively in this book.

Unit vectors are very useful for expressing other vectors; for example, we can express $\overrightarrow{a}$ and $\overrightarrow{a}$ of Figs. 3-7 and 3-8 as

$\overrightarrow{a}={a}_{x}\hat{i}+{a}_{y}\hat{j}$(3-7)

and $\overrightarrow{b}={b}_{x}\hat{i}+{b}_{y}\hat{j}$(3-8)

These two equations are illustrated in Fig. 3-14. The quantities
${a}_{x}\hat{i}$
and
${a}_{y}\hat{j}$
are vectors, called the vector components of
$\overrightarrow{a}$.
The quantities a_{x} and a_{y} are scalars, called
the *scalar components* of
$\overrightarrow{a}$
(or, as before, simply its a components).

**Adding Vectors by Components**

We can add vectors geometrically on a sketch or directly on a vector-capable calculator.A third way is to combine their components axis by axis.

To start, consider the statement

$\overrightarrow{r}=\overrightarrow{a}+\overrightarrow{b}$(3-9)

which says that the vector $\overrightarrow{r}$ is the same as the vector $\overrightarrow{a}+\overrightarrow{b}$. Thus, each component of $\overrightarrow{r}$ must be the same as the corresponding component of $\overrightarrow{a}+\overrightarrow{b}$:

r_{x} = a_{x} + b_{x} (3-10)

r_{y} = a_{y} + b_{y} (3-11)

r_{z} = a_{z} + b_{z} (3-12)

In other words, two vectors must be equal if their corresponding components are equal. Equations 3-9 to 3-12 tell us that to add vectors $\overrightarrow{a}$ and $\overrightarrow{b}$, we must (1) resolve the vectors into their scalar components; (2) combine these scalar components, axis by axis, to get the components of the sum $\overrightarrow{r}$; and (3) combine the components of to get itself. We have a choice in step 3. We can express $\overrightarrow{r}$ in unit-vector notation or in magnitude-angle notation.

This procedure for adding vectors by components also applies to vector
subtractions. Recall that a subtraction such as
$\overrightarrow{d}=\overrightarrow{a}-\overrightarrow{b}$
can be rewritten as an addition
$\overrightarrow{d}=\overrightarrow{a}+\overrightarrow{\mathrm{-b}}$.
To subtract, we add and by components,
$\overrightarrow{a}+\overrightarrow{\mathrm{-b}}$
to get
d_{x} = a_{x} - b_{x},
d_{y} = a_{y} - b_{y}, and
d_{z} = a_{z} - b_{z}, where
$\overrightarrow{d}=d{x}_{}\hat{i}+d{y}_{}\hat{j}+d{z}_{}\hat{k}$.(3-13)

**About the Authors**

David Halliday was an American physicist known for his physics textbooks, *Physics and
Fundamentals of Physics*, which he wrote with Robert Resnick. Both textbooks have
been in continuous use since 1960 and are available in more than 47 languages.

Robert Resnick was a physics educator and author of physics textbooks. He was born in Baltimore, Maryland on January 11, 1923 and graduated from the Baltimore City College high school in 1939. He received his B.A. in 1943 and his Ph.D. in 1949, both in physics from Johns Hopkins University.

The 10th edition of Halliday's Fundamentals of Physics, Extended building upon previous issues by offering several new features and additions. The new edition offers most accurate, extensive and varied set of assessment questions of any course management program in addition to all questions including some form of question assistance including answer specific feedback to facilitate success. The text also offers multimedia presentations (videos and animations) of much of the material that provide an alternative pathway through the material for those who struggle with reading scientific exposition.

Furthermore, the book includes math review content in both a self-study module for more in-depth review and also in just-in-time math videos for a quick refresher on a specific topic. The Halliday content is widely accepted as clear, correct, and complete. The end-of-chapters problems are without peer. The new design, which was introduced in 9e continues with 10e, making this new edition of Halliday the most accessible and reader-friendly book on the market.

A Reader says,"As many reviewers have noted, this is a great physics book used widely in university technical programs as a first course in technical physics, with calculus. I find it is the one book I start with when trying to understand physical concepts at a useful but basic level. It has broad coverage and is well written . To go beyond this book requires specialized books on each topic of interest (electromagnetics, quantum mechanics, thermodynamics, etc.)."

Reader Frank says, "The treatment is sound, thorough, and clear. I've owned the early editions of Halliday and Resnick for years. I'm very happy that I updated my library with this 10th edition. The topics are covered in a very logical order. The study features and worked examples are outstanding. Don't hesitate to buy this book! Reading it is awesome on the Kindle app on the iPad."

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