# Calc. Angle between radius vector point and positive x axis? What does that mean?

1. Jun 7, 2009

### tngo1206

Calc. Angle between radius vector point and positive x axis? What does that mean???

1. The problem statement, all variables and given/known data

The cartesian coordinates of a point in the xy
plane are x = −8.96 m, y = −1.75 m.

Calculate the angle  between the radius-
vector of the point and the positive x axis
(measured counterclockwise from the positive
x axis, within the limits of −180◦ to +180◦).

2. Relevant equations

arctan of (y/x) = angle/degree

3. The attempt at a solution

I did arctan of y/x = 11.051 incorrect....
I did the arctan of y/x + 180 and got 191.05 incorrect...
I did that same method - 180 = 168 ish still inccorrect...
then I did arctan of x/y + 180= 258 ish and still wrong..

any ideas what this question is asking for? or what the correct answer is??

2. Jun 7, 2009

### chriscolose

191.05 looks right but not within the limits of what they want. Try to express that another way.

3. Mar 21, 2010

### Ripcurl

I have this exact same problem with different numbers! (UT homework maybe? XD) I'm pretty stuck.

4. Mar 21, 2010

### D H

Staff Emeritus

5. Mar 21, 2010

### Ripcurl

Here is my problem:

The cartesian coordinates of a point in the xy plane are x = −8.83 m, y = −5.47 m. The distance between the point and the origin is 10.38700149. Calculate the angle  between the radius-vector of the point and the positive x axis (measured counterclockwise from the positive x axis, within the limits of −180◦ to +180◦). Answer in units of ◦.

I have no idea how he got his answer at all.

6. Mar 21, 2010

### D H

Staff Emeritus
A good start is to draw a picture. What does that tell you?

7. Mar 21, 2010

### Ripcurl

Okay, I drew a picture and I have a graph with a circle. I have the radius as 10.38700149 and I drew an arc (representing the angle) from the the positive x axis counterclockwise to the radius in the third quadrant.

8. Mar 21, 2010

### D H

Staff Emeritus
What trigonometric formulae do you think need to be used to solve this problem?

9. Mar 21, 2010

### Ripcurl

I understand that. XD That's why I asked. I wasn't expecting anyone to do anything for me. :3

Anyway, hmm, what formula? I can't say that I know. I mean, I'm thinking it has something to do with sin, but I'm not entirely sure.

10. Mar 21, 2010

### Ripcurl

Soh cah toa

11. Mar 21, 2010

### Ripcurl

sin=opposite/hyp.