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A uniform rectangle sign h=20.0cm high and w=11.0cm wide loses three of its four support bolts(at points p_1, P_3, and p_4) and rotates into the position as shown, with p_1 directly over p_3. It is supported by the bolt P_2, which is so tight it holds the sign from further rotation. Find the gravitational torque about p_2, if the mass of the sign is 5.0kg.

I couldn't get my image to appear but here is what is looks like: http://www.cramster.com/answers-apr-10/physics/physics-1-uniform-rectangular-sign-16-cm-high-andw-16-cm-wide_816165.aspx?rec=0

I am not really sure where to even begin on this problem so any input would be great! Thanks

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Welcome to Physics.SE! This looks rather like it violates the prohibition in our FAQ against "Do my homework" type questions. It there a concept that is eluding you here? – dmckee♦ Dec 11 '11 at 19:39
dmckee is right, we expect people asking homework questions to focus on the specific physical concept that is causing problems. If you can edit the question to do so, we'll be happy to reopen it. – David Zaslavsky♦ Dec 12 '11 at 0:14

closed as off topic by David Zaslavsky♦ Dec 12 '11 at 0:13

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1 Answer

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Since the problem ask about torque, try using its definition: $\tau = r\,F\,\sin\theta$. You should find out what are the appropriate $r$, $F$, and $\theta$ for your problem, of course.

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Well I have a few questions. When I work it out using the formula I get 4.9Nm and that's not right. The answer is 4.7Nm. So here are my calculations I was wondering if you saw anything wrong with them. F=mg=5kg*9.8m/s=49N r=.5*distance from p_2 to p_4 which using h and w I get to be 0.11413m. Then theta I think what I am supposed to do is shift the axis, means that I have the adj side to be 0.055m(half the base)/r. The I take the inverse cosine of that and get theta=61.19 degrees. But when I plug all these values in I am off by 0.2 in my answer and am not sure why. – ksmith Dec 11 '11 at 19:43
Check the angle you are trying to get. $\theta$ should be the angle between $r$ and $F$. Drawing the rectangle at scale might help you. – Arnoques Dec 11 '11 at 20:29

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