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Jetzt kostenlos anmeldenThere are two types of projections that you need to be aware of at this stage: **scalar projections** and **v****ector projections**.

Scalar projection simply gives the **length **in a particular **direction**. The result of this is a scalar that quantifies this amount. On the other hand, vector projection '**projects**' the length of one vector in the direction of another. One way to think of it is as if the 'shadow' of a vector is cast over the top of another vector.

We will start with the simplest to understand conceptually. The **scalar projection** of a vector finds how much, as a scalar, of the vector is in a particular direction. It is found using the **dot product **of the vector with the unit vector in the direction in question.

The scalar projection of a vector $\mathbf{x}$ onto unit vector $\hat{\mathbf{u}}$ is the scalar given by the **dot product**: $\mathbf{x}\xb7\hat{\mathbf{u}}$ or $>">\mathbf{x}\mathbf{,}\hat{\mathbf{}\mathbf{u}}$

Find the scalar projection of vector $\overrightarrow{\mathrm{OA}}$ in the horizontal direction.

**Solution**

The 'horizontal direction' is along the x-axis, therefore the unit vector we will be using is $\hat{\mathbf{u}}=\left[\begin{array}{c}1\\ 0\end{array}\right]$. Vector $\overrightarrow{\mathrm{OA}}$ in vector notation is $\left[\begin{array}{c}3\\ 4\end{array}\right]$.

Intuitively, you may have already realized that the scalar projection must be 3 since by definition $\overrightarrow{\mathrm{OA}}$ is comprised of 3 units in the horizontal direction (and 4 vertically).

We can show this using the dot product too: $>">\left[\begin{array}{c}3\\ 4\end{array}\right],\left[\begin{array}{c}1\\ 0\end{array}\right]$

Therefore, the scalar projection of $\overrightarrow{\mathrm{OA}}$ in the horizontal direction is equal to 3.

A **vector projection **is the projection of one vector onto another. It takes the length of one vector and projects it in the direction of another, creating a **new vector** with the direction of the second.

Vector **b** is known as *t**he* *projection of a in the x-axis direction *(figure 2)

In mathematical notation, this is written as ${\mathrm{Proj}}_{\mathit{c}}\left(\mathit{a}\right)$. We know that the vector $\underset{BA}{\to}$must be equal to $\mathit{a}-\mathit{b}=\mathit{a}-{\mathrm{Proj}}_{\mathit{c}}\left(\mathit{a}\right)$, therefore ${\mathrm{Proj}}_{\mathit{c}}\left(\mathit{a}\right)$ is such that $\mathit{a}-{\mathrm{Proj}}_{\mathit{c}}\left(\mathit{a}\right)$ is **orthogonal **to vector **c**.

This orthogonality is a property that is essential to finding the projection of **a **onto L. Remember the 'dot product'? Since $\underset{BA}{\to}$is orthogonal to line L, the 'dot product' of the two must be equal to **zero**. Using this information, we can derive a formula for the vector projection of **a**** **in the direction of **b**.

To understand fully what the projection vector does, it may be useful to see how to derive it. First, let's consider a vector **v** lying on line L**. **Since${\mathrm{Proj}}_{\mathit{v}}\left(\mathit{a}\right)$ is in the direction of L, we can write it as a scalar multiple of vector **v**.

**v**c is some constant

Using the dot product:

$(\mathbf{a}-{\mathrm{Proj}}_{\mathrm{L}}(\mathbf{a}\left)\right)\xb7\mathbf{v}=(\mathbf{a}-\mathrm{c}\mathbf{v})\xb7\mathbf{v}=0\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\mathbf{a}\xb7\mathbf{v}-\mathrm{c}\mathbf{v}\xb7\mathbf{v}=0\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\mathbf{a}\xb7\mathbf{v}=\mathrm{c}\mathbf{v}\xb7\mathbf{v}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\mathrm{c}=\frac{\mathbf{a}\xb7\mathbf{v}}{\mathbf{v}\xb7\mathbf{v}}$

Since the dot product of a vector with itself is equal to the length of that vector squared, we get:

$\mathrm{c}=\frac{\mathbf{a}\xb7\mathbf{v}}{|v{|}^{\mathrm{2}}}$.

And so, ${\mathrm{Proj}}_{\mathrm{L}}\left(\mathbf{a}\right)=\frac{\mathbf{a}\xb7\mathbf{v}}{{\left|\mathbf{v}\right|}^{2}}\mathbf{v}$.

The **vector projection** of vector **a **onto vector **v**** **is** ${\mathrm{Proj}}_{\mathrm{L}}\left(\mathbf{a}\right)=\frac{\mathbf{a}\xb7\mathbf{v}}{{\left|\mathbf{v}\right|}^{2}}\mathbf{v}$.**

We have vector $\mathbf{x}=\left[\begin{array}{c}6\\ 2\end{array}\right]$ and vector $\mathbf{v}=\left[\begin{array}{c}7\\ -6\end{array}\right]$. Find the projection of **x** onto **v**.

Using the formula: ${\mathrm{Proj}}_{\mathbf{v}}\left(\mathit{x}\right)=\frac{\mathbf{x}\xb7\mathbf{v}}{{\left|\mathbf{v}\right|}^{2}}\mathbf{v}$

${\left|\mathbf{v}\right|}^{2}={\left(\sqrt{{7}^{2}+{(-6)}^{2}}\right)}^{2}=85$

$\mathbf{x}\xb7\mathbf{v}=\left[\begin{array}{c}6\\ 2\end{array}\right]\xb7\left[\begin{array}{c}7\\ -6\end{array}\right]=42-12=30$

Therefore, ${\mathrm{Proj}}_{\mathbf{v}}\left(\mathit{x}\right)=\frac{30}{85}\mathbf{v}=\frac{6}{17}\left[\begin{array}{c}7\\ -6\end{array}\right]$.

${\mathrm{Proj}}_{\mathbf{v}}\left(\mathbf{x}\right)$ is represented by the blue arrow in the diagram below.

**Scalar projection**gives the**length**in which a vector is in a given**direction**- To find the scalar projection, use the
**dot product**of a vector with a**unit vector**in the direction in question - The formula for scalar projection of
**x**onto direction**u**is $\mathbf{x}\xb7\hat{\mathbf{u}}$ **Vector projection**projects the length of one vector into the direction of another- To find the projection of vector
**a**onto L, ${\mathrm{Proj}}_{\mathrm{L}}\left(\mathbf{a}\right)$ is such that $\mathbf{a}-{\mathrm{Proj}}_{\mathrm{L}}\left(\mathbf{a}\right)$ is orthogonal to line L - The projection of vector
**a**onto line L of vector**v**

Two examples of projection are scalar projection and vector projection.

Scalar projection is given by the dot product.

Both scalar projection and vector projection are done by applying their respective formula.

What are the two types of projections?

Scalar projection and vector projection.

Explain vector projection in words.

Vector projection 'projects' the length of one vector in the direction of another.

Explain scalar projection in words.

The **scalar projection** of a vector is how much the vector is in a particular direction.

(Revision) What is the difference between a scalar and a vector?

A scalar is a quantity with only magnitude (e.g. length). A vector has both magnitude *and *direction.

Define orthogonality.

If two vectors are orthogonal, they are perpendicular to each other (i.e.they are at right angles).

What is the dot product of orthogonal vectors?

Zero

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