Dynamics - determining an angle

[Femme_physics]

Homework Statement

http://img221.imageshack.us/img221/3696/abaod.jpg

In mechanism OAB, as described in the drawing, shaft OA rotates counterclockwise at an angular speed of 50 rad/sec.

Given:
OA = AB = R = 300mm

For the condition where angle Phi is 30 degrees, calculate:

A) The angular speed of the connecting rod AB
B) The speed of piston B

The Attempt at a Solution

Right now my confusion emanates from the velocities diagram of the solution manual. Notice that they've determined that the angle between Va and the X axis is 30 degrees. Based on what principle has this been determined? Yes, we do know that angles ABO and AOB are 30 degrees, but I don't see how that necessarily relates to the fact that the angle between Va and X axis is 30.

http://img202.imageshack.us/img202/1733/velocitiesdiagram.jpg

 

[I like Serena]

Can you say something about the angle between OA and v_A?

And the angle between OA and the x-axis (in its positive direction)?

 

[Femme_physics]

I'm sorry, I forgot a fact

OA = AB = R = 300mm

 

[Femme_physics]

Can you say something about the angle between OA and v_A?

Well, no, because I don't know at what angle do I pitch v_A. I don't know how they've come up with the fact it's 30 degrees.

And the angle between OA and the x-axis (in its positive direction)?

Well, that's a given to us, 120 degrees

 

[I like Serena]

Well, no, because I don't know at what angle do I pitch v_A. I don't know how they've come up with the fact it's 30 degrees.

And what if you consider that point A makes a circular motion around point O?

Well, that's a given to us, 120 degrees

Not that one - the other side (with the positive x axis)!

 

[Femme_physics]

And what if you consider that point A makes a circular motion around point O?

Great explanation. I can see clearly now the rain is gone :)

Not that one - the other side (with the positive x axis)!

Um, I guess I had a different idea of positive. Then it's 60 degrees! :) I think I'm able to unravel a lot now thanks to you. Hmm. Now I guess I can carry on, but class is starting in 50 minutes and I got to catch a ride. Thanks ILS for all the great help. :) I hope that now I'll be able to solve it without your godly touch directing me at the right course. :)