What is the angular acceleration of an arm

[Dusty912]

Homework Statement

what is the angular acceleration of the arm when theta equals 45 degrees.
v=2 m/s
picture is attached

Homework Equations

law of sites
law of cosines

The Attempt at a Solution

[/B]
so I used the law of sines to resolve side r as being 579.555 mm
now I now I need to take time derivative of theta somewhere using law of cosine or sines but I'm not really sure which one. also I don't know how to relate the 2 m/s to the problem

 

[Orodruin]

Hint: The velocity is telling you how fast the long side is increasing.

 

[Dr.D]

Write the vector expression describing the closed loop. Solve this for the angular orientation of the arm (it will appear in the vector components). Take the time derivative of that result.