# Finding angular velocity of telescopic arm

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1. Dec 4, 2013

### berdan

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

So ,here is the question : The structure is shown in the picture.
All I need is to find the angular velocity of telescopic arm BC ,in this current position.

http://img197.imageshack.us/img197/3954/w6e.JPG [Broken]

2. Relevant equations

Well,them simple ω=rXv/r$\bullet$r

3. The attempt at a solution

Here is my attempt,and I don't know why it is wrong .

I said that ω$_{bc}$=ω$_{b}$-ω$_{c}$

Then I calculated ω$_{b}$=rXv/r$\bullet$r , same with ω$_{c}$ .
The velocity of those points were simply r*ω of the joints.
I set the coordinates O point in the lower left corner . Anyway,I got the correct angular velocities next to that system of coordinates.
But when I tried to say ω$_{bc}$=ω$_{b}$-ω$_{c}$ , I got the wrong answer .
Why is that ?
What is the correct way to look at that problem ?