# 5. Dot Product (aka Scalar Product) in 2 Dimensions

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If we have any 2 vectors **P** and **Q**, the ** dot product** of **P** and **Q** is given by:

P`*`Q=|P| |Q|cosθ

where

|P|and|Q|are the magnitudes ofPandQrespectively, andθ is the angle between the 2 vectors.

The dot product of the vectors **P** and **Q** is also known as **the scalar product** since it always returns a **scalar value**.

The term dot product is used here because of the **•** notation used and because the term "scalar product" is too similar to the term "**scalar multiplication**" that we learned about earlier.

### Example 1

a. Find the dot product of the force vectors ** F _{1} **= 4 N and

**F**= 6 N acting at 40° to each other as in the diagram.

_{2}b. Find the dot product of the vectors **P** and **Q** if ** |P| **= 7 units and |**Q| **= 5 units and they are acting at right angles to each other.

The second example illustrates an important point about how scalar products can be used to find out if vectors are acting at right angles, as follows.

## Dot Product and Perpendicular Vectors

If 2 vectors act perpendicular to each other, the **dot product **(ie **scalar product**) of the 2 vectors has value **zero**.

This is a useful result when we want to check if 2 vectors are actually acting at right angles.

## Dot Products of Unit Vectors

For the unit vectors **i** (acting in the *x*-direction) and **j** (acting in the *y*-direction), we have the following dot (ie scalar) products (since they are perpendicular to each other):

i `*` j = j `*` i= 0

### Example 2

What is the value of these 2 dot products:

a. **i `*` i **

b. ** j `*` j **

## Alternative Form of the Dot Product

Recall that vectors can be written using scalar products of unit vectors.

If we have 2 vectors **P** and **Q** defined as:

P =ai+bj

Q=ci +dj,

where

a,b,c,dare constants;

iis the unit vector in thex-direction; and

jis the unit vector in they-direction,

then it can be shown that the **dot product** (**scalar product**) of **P** and **Q** is given by:

P `*` Q=ac+bd

### Example 3 - Alternative Form of the Dot Product

Find **P • Q** if

P= 6i+ 5jand

Q= 2i− 8j

Now we see another use for the dot product − finding the angle between vectors.

## Angle Between Two Vectors

We can use the dot product to find the angle between 2 vectors. For the vectors **P** and **Q**, the dot product is given by

P `*` Q=|P| |Q|cos θ

Rearranging this formula we obtain the cosine of the angle between **P** and **Q**:

`cos\ theta=(P * Q)/(|P||Q|)`

To find the angle, we just find the inverse cosine of the expression on the right.

So the angle θ between 2 vectors **P** and **Q** is given by

`theta=arccos((P * Q)/(|P||Q|))`

### Example 4

Find the angle between the vectors **P** = 3 **i** − 5 **j **and **Q** = 4 **i + **6 **j**.

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