I have a question about rotational movement in the vertical plane. I know I'm supposed to use polar coordinates to solve it, but since our professor only taught us how to derive the formulas and gave us no examples, I have no idea how to approach this problem.(adsbygoogle = window.adsbygoogle || []).push({});

A 227-g block B fits inside a small cavity cut in arm OA, which rotates in the vertical plane at a constant rate. When θ=180 degrees, the spring is stretched to its maximum length and the block exerts a force of 3.56 N on the face of the cavity closest to A. Neglecting friction, determine the values of θ for which the block is not in contact with that face of the cavity.

http://img23.photobucket.com/albums/v68/AngelOfMusic/1250.jpg [Broken]

What I've done so far:

At θ=180 degrees, the arm OA is vertical, with A pointing downwards. And I figure that:

ΣF=kx-mg in the upward direction. And this should be equal to ma(r), where a(r) = [tex]\ddot{r} - r\ddot\theta[/tex].

And this is equal to 3.65 N [?? I'm not sure on this one. ]

And this is where I'm lost, because the problem doesn't give any distances, it doesn't tell me the value of [tex]\dot\theta[/tex] or the value k for the spring. And we're expected to come up with a numerical answer of -53.2 degrees to 53.2 degrees.

Can anyone help me and point me to the right track?

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# Homework Help: Rotation in Vertical Plane

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