# Cartesian to Polar Vectors

1. Mar 25, 2010

Hi I pretty much can't get past the first chapter in my physics book until I master the vector representation of polar coordinates.

Every explanation I've read thus far has confused me, I keep thinking in Cartesian terms so I think it'd be great to convert a vector equation from cartesian to polar description and then differentiate (with somebodies help, which I need!).

I have a cartesian based vector,

$$r(t) = (2t^2)i + (3t - 2)j + (3t^2 - 1)k$$

I don't know how to go about converting it, could anybody give a helping hint?

Is it even possible? Every book I see polar coordinates mentioned have a $$coswt$$ etc... already mentioned.

Edit: Thinking about it, just a two dimensional vector would be easier on everyone!

$$r(t) = (2t^2)i + (3t - 2)j$$

Last edited: Mar 25, 2010
2. Mar 25, 2010

http://img716.imageshack.us/img716/4241/vec.jpg [Broken]

Hopefully this information will help somewhat!

Last edited by a moderator: May 4, 2017
3. Mar 26, 2010

$\overline{r} (t) = (2t^2)i + (3t - 2)j$

$convert \ to \ polar \ coordinates \ of \ the \ form \ \overline{r}(t) = \ r \hat{r} \ then \ differentiate!$

1: First we want to calculate |r| = r.

$r \ = \ | \overline{r} | \ = \ \sqrt{(2t^2)^2 + (3t - 2)^2} \ = \sqrt{4t^4 + 9 t^2- 12t + 4}$

2: We know our unit vector will be;

$\hat{r} \ = \ cos \theta \ + \ \sin \theta$

3. Calculate θ!

$\theta \ = \ \arctan( \frac{y}{x} ) \ = \ \arctan(\frac{3t - 2}{2t^2} )$

4. We see that $\hat{r}$ must be;

$\hat{r} \ = \ cos \theta \ + \ \sin \theta \ = \ \cos [\arctan(\frac{3t - 2}{2t^2} )] \ + \ \sin [\arctan(\frac{3t - 2}{2t^2} )]$

5. Put the position vector all together, (all are equivalent).

$\overline{r}(t) = \ r \hat{r}$

$\overline{r}(t) \ = \ [ \sqrt{4t^4 + 9 t^2- 12t + 4} ] \hat{r}$

$\overline{r}(t) \ = \ [ \sqrt{4t^4 + 9 t^2- 12t + 4} ] [\ cos \theta \ + \ \sin \theta ]$

$\overline{r}(t) \ = \ ( \sqrt{4t^4 + 9 t^2- 12t + 4} ) (cos \theta ) \ + \ ( \sqrt{4t^4 + 9 t^2- 12t + 4} ) (sin \theta )$

$\overline{r}(t) \ = \ ( \sqrt{4t^4 + 9 t^2- 12t + 4} ) ( \cos [\arctan(\frac{3t - 2}{2t^2} )] ) \ + \ \sin ( \arctan(\frac{3t - 2}{2t^2} ) ] )$

$\overline{r}(t) \ = ( \sqrt{4t^4 + 9 t^2- 12t + 4} ) ( \cos [\arctan(\frac{3t - 2}{2t^2} ) ] ) \ + ( \sqrt{4t^4 + 9 t^2- 12t + 4} ) ( \ \sin [\arctan(\frac{3t - 2}{2t^2} ) ] )$

Alright, so I've set up the position vector as best I can. I'm assuming there's nothing wrong so I'll now take the derivative to find the velocity vector.

6. Using;

$\overline{r}(t) \ = \ [ \sqrt{4t^4 + 9 t^2- 12t + 4} ] [\ cos \theta \ + \ \sin \theta ]$

I'll differentiate to obtain an equation of the form;

$\overline{v} (t) \ = \ \frac{d \overline{r} }{dt} \ = \ r \frac{d \hat{r}}{dt} \ + \ \frac{dr}{dt} \hat{r}$

Here I go!

$\frac{d \overline{r} }{dt} \ = \ ( \sqrt{4t^4 + 9 t^2- 12t + 4} ) (- sin \theta \frac{d \theta} {dt} \ + \ cos \theta \frac{d \theta}{dt} ) \ + \ ( \frac{16t^3 \ + \ 18t \ - \ 12}{2 \sqrt{4t^4 + \ 9t^2 - 12t + 4} } ) (\ cos \theta \ + \ \sin \theta )$

$\frac{d \overline{r} }{dt} \ = \ ( \sqrt{4t^4 + 9 t^2- 12t + 4} ) (- sin \theta \ + \ cos \theta ) \frac{d \theta}{dt} \ + \ ( \frac{16t^3 \ + \ 18t \ - \ 12}{2 \sqrt{4t^4 + \ 9t^2 - 12t + 4} } ) (\ cos \theta \ + \ \sin \theta )$

I didn't dare try to bring in the arctan and it's craziness into this one, (yet...).

7. Look for patterns!

Well,

$r \ = \ \sqrt{4t^4 + 9 t^2- 12t + 4}$

$\dot{r} \ = \ ( \frac{16t^3 \ + \ 18t \ - \ 12}{2 \sqrt{4t^4 + \ 9t^2 - 12t + 4} } )$

$\hat{r} \ = \ cos \theta \ + \ \sin \theta$

$\frac{ d \hat{r}}{dt} \ = \ ( - \sin \theta \ + \ \cos \theta ) \frac{ d \theta}{dt}$

We'll set;

$\hat{ \theta } \ = \ (- sin \theta \ + \ cos \theta )$

&

$\dot{ \theta} \ = \ \frac{d \theta}{dt}$

to get

$\frac{ d \hat{r}}{dt} \ = \ \hat{ \theta } \dot{ \theta}$

8. Write the final, simplified equation for velocity.

$\overline{v} (t) \ = \ r \dot{ \theta} \hat{ \theta } \ + \ \dot{r} \hat{r}$

Comparing the result of 8. with the result of 6. using 7. to be sure we've labelled everything correctly I'd say we have taken a deceptively simple equation in cartesian form, converted it to it's equivalent polar coordinate description & take the derivative of this to obtain the velocity.

Q.E.D.

$$\alpha :$$Have I missed anything?

$$\beta :$$ Have you ever actually had to do a calculation like this??