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Old Jul 18th 2013, 07:09 AM   #1
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Three forces and a couple are acting on a signboard. Represent these forces by: a) A

Three forces and a couple are acting on a signboard. Represent these forces by:
a) A force-couple system acting at point B
b) A single force along line CD

Illustration: http://imageshack.us/photo/my-images/594/ey7r.jpg/
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Old Jul 18th 2013, 11:23 AM   #2
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Originally Posted by iannedrs View Post
Three forces and a couple are acting on a signboard. Represent these forces by:
a) A force-couple system acting at point B
b) A single force along line CD

Illustration: http://imageshack.us/photo/my-images/594/ey7r.jpg/
My definition of "couple" must not be what yours is. Can you please let me know what your definition is?

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Old Jul 18th 2013, 02:01 PM   #3
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iannedrs: for part(a) what have you been able to do so far? Do you know how to translate a couple from one point to another, and how a force acting at one point is equivalent to that same force plus a couple acting at another point?

As for (b) - in general it's not possible to represent a set of forces and moments (couples) as a single force acting along a given line. You may be able to repesent the system as a single force, but it won't be along line CD.
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Old Jul 22nd 2013, 04:03 AM   #4
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vague recollections

I believe (remembering from lessons of long-long ago) that a simple 2D system of forces and couples, as described,
can always be redefined using one force and one couple about any given point in the 2D plane.
The force and couple will be different, depending on which point in the plane the couple is chosen to act about.
Pushing my memory even further, I think it is the case that (for any 2D combination of Forces and Couples as described)
there will always be one (and only one) couple point and force vector for which the couple is zero.
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