# R^ what does it mean?

1. Apr 10, 2005

### Leonidas

I was watching some Public television at 2:30 AM (it's when all the best programming is on)... and they were doing this bit on old astronomers and the mathematics behind it...

they didn't go into detail, and i understood everything that was spoken aloud... but they would flash equations in the backgrounds that I could ALMOST follow...

the main thing i didn't understand is they had an r with a ^ on top of it....

i assumed it had something to do with the radius... its not the average radius of an ellipse, is it?

anyways, if you feel like it, satisfy my curiosity...

also, if anyone wants to explain eccentricity of ellipses and how angular momentum is calculated, i'd be interested as long as they were in terms a high school calculus student could understand.

thanks! :shy:

2. Apr 10, 2005

### SpaceTiger

Staff Emeritus
Do you mean:

$$\hat{r}$$

That usually refers to a unit vector in the direction of something's radius. A unit vector is a vector of magnitude equal to one.

3. Apr 10, 2005

### dextercioby

A more proper notation would be

$$\hat{e}_{r}$$...

Daniel.

4. Apr 10, 2005

### SpaceTiger

Staff Emeritus
Do you have a reason for saying this? I have plenty of textbooks that use the $$\hat{r}$$ convention.

5. Apr 10, 2005

### Leonidas

so in an equation about a heavenly body revolving around another heavenly body,

r^ basically says that it is defining the speed as 1 and the direction as around something else?

what does "in the direction of the radius" mean?

6. Apr 10, 2005

### dextercioby

What about the other 2 unit vectors in the spherical coordinates ?

Daniel.

7. Apr 10, 2005

You're both wrong. It's $$\hat{i}_r$$ ... !

8. Apr 10, 2005

### dextercioby

Actually the dot it's over the hat...

Daniel.

9. Apr 10, 2005

### SpaceTiger

Staff Emeritus
In the dynamics book I have by my side, they're denoted $$\hat{\theta}$$ and $$\hat{\phi}$$...or is that supposed to be a response to the OP?

10. Apr 10, 2005

### Leonidas

um... i still don't understand what it means.

11. Apr 10, 2005

### SpaceTiger

Staff Emeritus
Yeah, sorry.

Imagine that you have an object around which you're basing your coordinate system. In orbital problems, it's often the sun. If you draw an arrow from the sun to any point in that coordinate system, that arrow is called the point's "radius vector". The earth's "radius vector" is an arrow pointing towards it from the sun. Now, $$\hat{r}$$ is a unit vector, so that means that it must have a length equal to one. So, to get the earth's unit radius vector, you just expand or contract the "radius vector" until it's equal to one. Its direction will be the same, but its length (or magnitude) will be different.

12. Apr 10, 2005

### Leonidas

lol thanks, that makes a bit more sense... i think i'm in over my head here, though :-)

So when you say expand or contract the radius vector... what do you mean?

If you think its too hard to explain in terms i'll understand, i can wait till college :-)

-thanks!

13. Apr 10, 2005

### dextercioby

Sorry,SpaceTiger,i was degenerating it into "conventions and likes vs. dislikes"...You know what i think,though...

Daniel.

14. Apr 10, 2005

### SpaceTiger

Staff Emeritus
Let me put it in terms of an equation, since this is hard to describe in words:

$$\hat{\bold r}=\frac{\vec{\bold r}}{|\vec{\bold r}|}$$

What that means is that the unit vector is the radius vector (did you understand what I meant by that?) divided by its length. Unit vectors are generally used to specify a direction. If I want to specify a vector for something moving at speed v away from the sun, I just write:

$$\vec{\bold v}=v\hat{\bold r}$$

15. Apr 10, 2005

### Leonidas

I'm not sure I do understand what radius vector means...

I assumed it meant a measurement which coupled the length of the radius with the direction it's rotation.

"If I want to specify a vector for something moving at speed v away from the sun"

Away from the sun? do you mean around the sun?

16. Apr 10, 2005

### Zorodius

Imagine you have the vector <2, 4>. The vector <1, 2> points in exactly the same direction as the first vector, but it is shorter. The vector <4, 8> points in the same direction as those other two vectors, but it is longer.

All three of these vectors are pointing the same way, but they differ in that some of them are longer or shorter than others. You could think of this as taking a vector, and expanding it (making it longer without changing the direction it's pointing in) or contracting it (making it shorter without changing the direction it's pointing in.)

If, after expanding or contracting the vector, the length of the vector was 1, you produced a unit vector.

17. Apr 10, 2005

### Leonidas

alright thanks everyone! i'm not sure i understand yet, but I'll look into it a little bit more... maybe ask my math and physics teachers... it'll probably be easier to explain in person.

:-) thanks, and goodnight.

18. Apr 10, 2005

### whozum

A vector in simple terms is basically an arrow. It points from one place to another. As spacetiger was saying, a radius vector is an arrow pointing from a certain place (the sun) to whereever you want it to (the earth). This is noted by the r with the hat on it. A unit vector is a vector which is one unit long. Say the distance frmo the earth to the sun is 5000 km (this is way wrong, just an example), then your vector will be 5000km long, but a unit vector is just 1m long. So what you do is you point your arrow in the direction of the earth (from the sun) and just squash it until its 1m long. However it still maintains its direction (from earth to sun), its just a lot shorter.

19. Apr 10, 2005

### SpaceTiger

Staff Emeritus
Have you ever worked with vectors before? This might be tough to explain in words if you haven't, but try this. If you have graph paper, draw a pair of coordinate axes on it. If not, draw a grid, like this.

Now, look at the point in that diagram. Do you know how to get its x and y coordinates? To get the coordinates, just count the number of squares over it is in the x and y directions. In this case, it's 7 squares in the x direction and 4 in the y direction, so its coordinates are (7,4).

Now try drawing an arrow to the point. This is called a "vector". The problem now is that we want to describe that vector. One way to do it is in terms of unit x and y vectors. They're called $$\hat{x}$$ and $$\hat{y}$$. Graphically, you can describe them by drawing an arrow from (0,0) to (1,0) and an arrow from (0,0) to (0,1). The first is the x unit vector and the second is the y unit vector. It turns out that you can represent the arrow you drew first (the one to the point) by a linear combination of these vectors. That is:

$$\vec{\bold v}=7\hat{\bold x}+4\hat{\bold y}$$

Try adding seven of the x unit vectors and four of y ones to verify. These are called "Cartesian coordinates". It turns out that you can also express this point in "polar" coordinates (or spherical, in three dimensions). To do that, instead of measuring x and y, you measure the distance from the center of the coordinate system (that's (0,0) in cartesian coordinates) and the angle from the x axis. For polar coordinates, the unit vectors are $$\hat{\bold r}$$ and $$\hat{\bold \theta}$$, an example of which can be shown here .

Last edited: Apr 11, 2005
20. Apr 11, 2005

### Nylex

No, he meant away from the Sun in the same way that you'd say you were walking away from a building.