Calc Angle b/w Radius Vector Point & +x Axis: What Does That Mean?

[tngo1206]

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

Homework Statement

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◦).
Answer in units of ◦.

Homework Equations

arctan of (y/x) = angle/degree

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??

 

[chriscolose]

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

 

[Ripcurl]

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

 

[D H]

Read chriscolose's answer. Is 191.05 between -180 and +180?

 

[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.

 

[D H]

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

 

[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.

 

[D H]

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

Our job here is to help you do your own homework; we do not do your homework for you.

 

[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.

 

[Ripcurl]

Soh cah toa

 

[Ripcurl]

sin=opposite/hyp.
cos=adjacent/hyp
tan=opposite/adjacent

 

[D H]

Then maybe your drawing should have a triangle that you can make sense of rather than an arc that you cannot make sense of.