# Dynamics Help Needed Relative velocities problem

1. Sep 30, 2011

### coltin.walsh

Dynamics Help Needed!! Relative velocities problem

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

http://www.flickr.com/photos/55153239@N03/6199846646/

2. Relevant equations

I have 3 unknowns: Va, Vb, and Thetha; but only two equations

Vb(cos(theta)) - Va(cos30) = 5.6cos70
Vb(sin(theta)) - Va(sin30) = 5.6sin70

I need a third equation???!?!

3. The attempt at a solution

To get the two above equations, I took into account:

Vb/a = Vb - Va

Then I broke each vector into components. I end up with the two above equations. But I don't know where to go from here. I need a third equation either relating theta to Va or Vb, or a cable constraint equation, etc. I need some help.

Last edited by a moderator: Apr 26, 2017
2. Oct 1, 2011

### coltin.walsh

Re: Dynamics Help Needed!! Relative velocities problem

Is it right to use the cable constraint equation of: 2Xa+3Xb=constant.
then taking the derivative to get: 2Va+3Vb=0? Or is that not correct?

3. Oct 1, 2011

### rude man

Re: Dynamics Help Needed!! Relative velocities problem

(I haven't done this problem, so this is truly just a suggestion, but I think it would lead to your answer).

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