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Old Nov 7th 2016, 12:16 PM   #1
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Moments Help!

Hello everyone,

I can do part a easily, but I have no idea how to do part b. Can I have a step-by-step of how to do it - I know that you have to take moments about either A or B, and I know that the horizontal force doesn't have to be considered in the problem either (due to it being perp. to the weight).

Can anyone help?
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Old Nov 7th 2016, 01:06 PM   #2
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Your free body diagram should include a horizontal force on the left end of the pole at A, and both horizontal and vertical forces at B. From sum of vertical forces = 0 you have Bv = W, and from sum of horizontal forces =0 you have Ah = Bh. Then take the moment about point B and set it to zero. You now have three equations in 4 unknowns; the 4th equation you need is from the fact that at the contact point B the reaction force acts perpendicularly to the pole - in other words Bh/Bv = tan (theta). Can you take it from here?
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Old Nov 7th 2016, 01:13 PM   #3
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Thanks for the help - the bit that I'm having the most trouble with is the moments part itself. Would I put the length of the rod inside the skip as x, or is there a way of working it out?
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Old Nov 7th 2016, 01:34 PM   #4
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Regarding the moments about point B, the horizontal force at A has a moment arm of 2m x sin(theta), do you see why that is? And the weight of the pole acts vertically and has a horizontal moment arm from B to the center of mass of 2.5m x cos(theta) - 2m.
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Old Nov 7th 2016, 02:11 PM   #5
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So does 2 sin (theta) = 0.5 cos (theta) ?

Last edited by electrogeek; Nov 7th 2016 at 02:22 PM.
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Old Nov 7th 2016, 04:39 PM   #6
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Originally Posted by electrogeek View Post
So does 2 sin (theta) = 0.5 cos (theta) ?
No.

$\displaystyle B_h \ \times \ 2 \sin \theta = W(2.5 \cos \theta -2)$

Maybe the attached will help you visualize this. Then use the fact that $\displaystyle B_h/B_v = \tan \theta$, so the $\displaystyle B_h$ on the left hand side can be replaced by $\displaystyle B_v \tan \theta = W \tan \theta$. Then it's a matter of some manipulation to solve for cos theta.
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Last edited by ChipB; Nov 8th 2016 at 08:51 AM.
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Old Nov 8th 2016, 07:58 AM   #7
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I understand - thank you so much for the diagram! Everything has jut clicked into place! Thanks for all the help.

Last edited by electrogeek; Nov 8th 2016 at 08:20 AM.
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