# Math Help - vectors!!

1. ## vectors!!

hello!!

i'm having a little trouble converting between vector forms!!
so how would you convert the vector V = (3,40 degrees) into cartesian form
and convert v = 2i + 5j into polar form...

thanks!!

2. Hello, sir nerdalot!

How would you convert the vector $v \:= \:(3,\,40^o)$ into cartesian form
and convert $v \:= \:2i + 5j$ into polar form?

If you sketch this diagram, the pieces should come together.
Code:
        |
|     *
|    /|
| r / |
|  /  |y
| /   |
|/θ   |
- - * - - + - -
|  x
We can see that: . $\begin{array}{c}x \: =\: r\cos\theta \\ y \: =\: r\sin\theta\end{array}$

. . . and that: . $\begin{array}{ccc}r & = & \sqrt{x^2+y^2} \\ \theta & = & \tan^{-1}\!\left(\frac{y}{x}\right)\end{array}$

Then use those conversion formulas . . .

$\begin{Bmatrix}x & = & 3\cos40^o & \approx & 2.3 \\
y & = & 3\sin40^o & \approx & 1.9\end{Bmatrix}\qquad (3,\,40^o)\;\Rightarrow\;2.3i + 1.9j$

$\begin{Bmatrix}r & = & \sqrt{2^2+5^2} & = & \sqrt{29} \\
\theta & = & \tan^{-1}\!\left(\frac{5}{2}\right) & \approx & 68.2^o\end{Bmatrix}\qquad2i + 5j\;\Rightarrow\;\left(\sqrt{29},\,68.2^o\right)
$

3. thanks for that....

but how would i find the angle between these two vectors???
i thought about trying to use the cosine/sine rule but found that there wasn't enough information to do this.

4. Originally Posted by sir nerdalot
thanks for that....

but how would i find the angle between these two vectors???
i thought about trying to use the cosine/sine rule but found that there wasn't enough information to do this.

use the dot product formula

Recall: If $\bold {a,b}$ are two vectors, then $\boxed { \bold {a} \cdot \bold {b} = |\bold {a}||\bold {b}|\cos \theta }$ where $\theta$ is the angle between them.

you have enough information to solve that formula for $\theta$

do you remember how to take the dot product?

5. Hello, sir nerdalot!

I thought about trying to use the cosine/sine rule . . . . Good thinking!
but found that there wasn't enough information to do this.
. .
Well, there is . . . but it takes some Olympic-level gymnastics.

Given two vectors $\vec{u}$ and $\vec{v}$, let $\theta$ be the angle between them.
Code:
                  Q
*
*  \
v  *     \
*        \ w
*           \
* θ            \
* * * * * * * * * *
O          u         P
We have: . $\vec{u} \,= \,\overrightarrow{OP},\;\;\vec{v} \,=\, \overrightarrow{OQ},\;\;\theta \,= \,\angle POQ$

. . If $\vec{w} \,= \,\overrightarrow{PQ}$, then: . $\vec{w} \:= \:\vec{u} - \vec{v}$

Using the Law of Cosines, we have: . $|w|^2 \;=\;|u|^2 + |v|^2 - 2|u||v|\cos\theta$ .[1]

A small detour:

. . $|\vec{w}|^2 \;=\;|\vec{u} - \vec{v}|^2 \;=\;(\vec{u}-\vec{v})\cdot(\vec{u}-\vec{v}) \;=\;\vec{u}\cdot\vec{u} - \vec{u}\cdot\vec{v} - \vec{v}\cdot\vec{u} + \vec{v}\cdot\vec{v}$

. . $|w|^2\;=\;|\vec{u}|^2 - 2(\vec{u}\cdot\vec{v}) + |\vec{v}|^2$ .[2]

Substitute [2] into [1]: . $|\vec{u}|^2 - 2(\vec{u}\cdot\vec{v}) + |\vec{v}|^2 \;=\;|\vec{u}|^2 + |\vec{v}|^2 - 2|\vec{u}||\vec{v}|\cos\theta$

This simplifies to: . $2|\vec{u}||\vec{v}|\cos\theta \:=\:2(\vec{u}\cdot\vec{v})\quad\Rightarrow\quad\b oxed{\cos\theta \;=\;\frac{\vec{u}\cdot\vec{v}}{|\vec{u}||\vec{v}| }}$ . . . . There!

6. coolies and so i substitute the two sides into u and v

thanks all!!!