In this explainer, we will learn how to find the components of a given two-dimensional vector.
Vectors can be represented in a suitable space by a line segment with a specific length, or magnitude, and direction. This means that we can think of vectors as defining movement: traveling from a starting point in a given direction for a specified distance. Vectors can also represent, for example, force or acceleration.
We can also define a two-dimensional vector using its horizontal and vertical components by writing it either in the form or in the form . Here, we will discuss these horizontal and vertical components and introduce the and unit vector notation.
Any two-dimensional vector has two components. When a vector is represented graphically by a line segment on a graph, represents the horizontal difference between the -coordinates of its endpoints, and represents the vertical difference between the -coordinates. The numbers and can also be thought of as representing movement in the horizontal and vertical directions.
Example 1: Finding the Horizontal and Vertical Components of a Vector
Consider the vector for and . Write in the form .
To find the horizontal and vertical components of , that is, to find and such that , we can consider the horizontal and vertical distances from to . Another way to look at this is to find how far we have to move in the horizontal direction () and the vertical direction () to get from to .
The horizontal distance is equal to the difference between the -coordinates; this is . The vertical distance is equal to the difference between the -coordinates, which is .
Hence, we can write the vector from to as
How To: Writing a Vector as the Sum of Unit Vectors
To write a vector where and in the form , first calculate the difference between the -coordinates to obtain and then calculate the difference between the -coordinates to obtain :
In this form, represents the -component, or the horizontal distance, and represents the -component, or the vertical distance.
This notation can be extended to any number of dimensions. If we had a three-dimensional coordinate system with -, -, and -coordinates, then we would add a third number to represent the -component: .
Example 2: Finding the Horizontal and Vertical Components of a Vector
Consider the vector in the diagram.
- What are the coordinates of its terminal point?
- What are the coordinates of its initial point?
- What are the components of the vector?
When a vector is represented on a plane, the terminal point of the vector is the point where the head of the arrow is. We can think of this as where the vector is pointing. From the diagram, we can see that is the point where the arrow ends.
Similarly, the initial point is the point from which the arrow starts. Hence, by looking at the diagram, we can see that is the initial point.
The first component of the vector can be found by considering the difference between the -coordinates of the terminal and initial points; that is, the first component (or, equivalently, the -component) of the vector is . As for the second component, it is the difference in the -coordinates of the terminal and initial points. Hence, the second component (or, equivalently, the -component) is given by . When we write vectors in component form, we use the notation .
Next, we define two special vectors which have length 1. The vector represents moving a distance of positive one in the -direction, and the vector represents moving a distance of positive one in the -direction.
Definition: The Unit Vectors
Using the notation we described earlier, we can define where represents moving one unit in the -direction and no units in the -direction and represents moving no units in the -direction and one unit in the -direction.
It is important to note that these special vectors do not have to start at the origin. These unit vectors just describe moving a distance of one in either the horizontal or the vertical direction; the start point can change.
For example, as seen in the figure below, the unit vector can represent moving from to since the -coordinate increases by one and the -coordinate does not change.
The unit vector can represent moving from to since the -coordinate does not change and the -coordinate increases by one.
Now, we will investigate how to add and subtract with these unit vectors.
The vector represents moving along copies of . We can write which means that we move units in the horizontal direction and no units in the vertical direction.
The vector represents moving along copies of . We can write which means that we move units in the vertical direction and no units in the horizontal direction.
For example, shown in orange in the figure below is the vector .
Once we have vectors of the form and we can add them together to describe any vector in the form .
Example 3: Writing Vectors as the Sum of Unit Vectors
Using that each square in the grid has length 1, write the vector in the form and then in the form .
From the start point , we move units in the horizontal direction (which represents the vector ) and then move units in the vertical direction (which represents the vector ) to get to the point .
The vector , which represents traveling directly from to , is then the sum of these unit vectors.
How To: Writing a Vector in Component Form
In general, if we travel from a starting point to a finishing point then this describes a vector which represents traveling a distance of in the -direction and then a distance of in the -direction.
We can write this vector in two ways: or
Using the vectors and allows us to describe the vector in terms of the number of horizontal and vertical steps of length one we would have to take to get from the start point to the end point.
Note that negative coefficients of and represent moving in the opposite direction.
For example, the vector , which represents traveling two negative units in the -direction and three negative units in the -direction, as seen in the figure above, can be written as or
Sometimes, we have to solve word problems with vectors where the main skill is interpreting the wording of the question and translating it into mathematics.
Example 4: Word Problems Involving Components of Vectors
A body moved 190 cm due east, where and are two unit vectors in the east and north directions respectively. Express its displacement in terms of the two unit vectors and .
We have been told that represents the easterly direction and the northerly direction. The body moved east. Therefore, the vector expressing this displacement will have no northerly component. Therefore, the coefficient of will be zero. We know that it moved 190 cm in the easterly direction; therefore, the coefficient of will be 190. Therefore, the displacement of the body can be written as
Example 5: Adding Vectors Given as Sums of Unit Vectors
Let and . Find .
When we add vectors, we add the -components together (to calculate the total horizontal movement) and then add the -components together (to calculate the total vertical movement). So,
We can also consider adding these vectors graphically. From our starting point we move in the -direction and in the -direction. This represents the vector . From this point, we move in the -direction and in the -direction. This represents the vector . This is shown in the figure on the left.
Then, the sum of and is the vector which describes the movement from the original start point to the end point. As the figure on the right shows, the vector is equal to .
Example 6: Subtracting Vectors Given as Sums of Unit Vectors
Let and . Find .
In the same way as when adding, to subtract vectors we consider the and components separately.
For the components, we have and for the components we have . Writing this in full, we get
- There are two ways to write vectors when considering horizontal and vertical components: in addition to writing vectors in the form , we can describe them using sums of the unit vectors and . The -component is the coefficient of and the -component is the coefficient of . So, we can write the vector as .
- When we add or subtract vectors, we have to combine the components and the components separately.
- Given two vectors and
find , add the horizontal
components and the vertical components separately.
So, , where Written formally, we calculate the sum as follows: Similarly, we can find the difference of the two vectors by taking the difference of their horizontal and vertical components. Hence, , where Equivalently, this can be written as