# Vector

Image 1: Image of a vector
Modified Image 1

Vectors are quantities that are specified by both a magnitude of length and a direction. Perhaps the simplest vector is a Euclidean vector, represented by an arrow in Euclidean space. Refer to Image 1. This arrow has a length and points in some direction.

The length of a vector, which is measurable, is called the magnitude. The magnitude is denoted by $\left| \vec A \right|$. The magnitude of a vector is a scalar quantity, a numerical value with no direction.

Image 1 is a vector. However, Image 1 is not very useful because it's unclear where it is positioned. To use vectors, we need to introduce the coordinate system. Assigning x and y coordinates to a vector allows us to be more precise when we talk about the location of a vector. Refer to Modified Image 1 on the left. In this updated image, we now know the x and y coordinates. Our vector is 6 units in the x direction and 8 units in the y direction.

Image 2: Unit Vectors (i, j, k)

Another way to define the position of a vector is by using unit vectors, which have length 1. A unit vector is denoted by a lowercase letter with a hat above it, like so, $\hat{a}$. It is pronounced as a-hat. Unit vectors correspond to the x-y-z coordinate system. $\hat{i}$ points along the x-axis $\hat{j}$ points along the y-axis and $\hat{k}$ points along the z-axis. This notation is used in physics, engineering and linear algebra. In order to give a vector's position using unit vectors, we write it as a combination of unit vectors that are placed along the the coordinate axes.

We already described our vectors position just using coordinate points. We can also use unit vector. A unit vector initially has a length of one, but we can change the magnitude by stretching or shrinking the unit vectors. Imagine the unit vector is a rubber substance that can stretch and shrink. You can learn more of this in the graphical introduction to vectors. Refer to Modified Image 1. To express this vector using unit vectors, we would need a unit vector with a magnitude of 6 going in the x direction and 8 a unit vector with a magnitude of 8 going in the y direction.

## Labeling Vectors

Vector (3, 2): Click to enlarge

When we have a vector quantity we put an arrow on top of the labeling letter to remind us that it is a vector. It looks like so $\vec A$. One way of writing vectors is by components, like this: $\left \langle a_1, a_2, a_3 \right \rangle$. The components of a vector can also be written as: $\begin{bmatrix} 3 \\ 2\\ 0\\ \end{bmatrix}$. This vector has an x-component of 3, a y-component of 2, and a z-component of 0. This is shown in the image on the right.

With $\left \langle a_1, a_2, a_3 \right \rangle$ as any numerical values we can also write any vector in terms of standard unit vectors: $\vec A = a_1 \hat{i} + a_2 \hat{j} + a_3 \hat{k}$. If we are given $\vec A = 3 \hat{i} + 3 \hat{j} + 2 \hat{k}$ we know that vector A is 3 units in the x and y direction and 2 units in the z direction. This is shown in the image below.

Vector (3,3,2): Click to enlarge

If a vector is still a bit abstract to you then think of a compass. The arrow has a certain length, this is our magnitude, and it points in any direction (north, south, east, and west).

'Head to tail' addition of two vectors: (3, 2) + (1,-1) = (4, 1): Click to enlarge

When two vectors are added, the sum can be found by placing the two vectors 'head to tail' and finding the coordinate they reach. When a vector is multiplied by a scalar several things may occur:

• When multiplied by a number with absolute value greater than one, the vector stretches. (Figure 1)
• When multiplied by a number with absolute value less than one, the vector shrinks. (Figure 2)
• When multiplied by a negative number, the vector reverses direction. (Figure 3)

Many mathematical and physical entities can be represented by vectors. For example, an object's velocity can be represented by a 'velocity vector', where each component represents the object's speed in a certain direction. Gravity can also be represented by a vector: the strength of gravity's pull corresponds to the magnitude of the vector, and the direction of the pull corresponds to the direction the vector points in. Vector Fields are another important application of vectors.

 Figure 1: Stretching a vector A Figure 2: Shrinking a vector A Figure 3: Stretching and reversing the direction of a vector A

For vectors $\vec{A},\vec{B},$ and $\vec{C}$ and numbers $d$ and $e$,

The following equalities hold for all vectors and scalars:

• $\vec{A}+\vec{B} = \vec{B} + \vec{A}$ (Commutivity of vector addition)
• $\vec{A}+(\vec{B}+\vec{C}) = (\vec{A}+\vec{B})+\vec{C}$ (Associativity of vector addition)
• $d(\vec{A}+\vec{B}) = d\vec{A} + d\vec{B}$ (Distributivity of scalars)
• $d(e\vec{A}) = (de)\vec{A}$ (Associativity of scalar multiplication)
• the existence of a zero vector (additive identity), $\vec{0}$, such that $\vec{A} + \vec{0} = \vec{A}$

The standard algebraic representation of vectors is in terms of components, although not all vectors can be expressed this way:

$\vec{A} = (a_1, a_2, ... , a_k)$
$\vec{B} = (b_1, b_2, ... , b_k)$

Vectors in this form are added by components:

$\vec{A} + \vec{B} = (a_1, a_2, ... , a_k)+(b_1, b_2, ... , b_k)=(a_1+b_1, a_2+b_2, ... ,a_k + b_k)$

Multiplying a vector by a number, known as a scalar, means multiplying each component by that number:

$d\vec{A} = (da_1, da_2, ... , da_k)$