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Momentscalculate centre of mass 
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#1
Dec808, 04:14 PM

P: 112

1. The problem statement, all variables and given/known data
A woman and a man support a nonuniform plank, AB of mass 10kg and length 4m. The woman holds the plank 1m from A and the man holds the plank at B. The vertical reaction provided by the woman is 56N find; A) the vertical reaction force provided my the man b) The position at which at which the weight of the plank acts measured from A. 2. Relevant equations 3. The attempt at a solution I worked out the vertical reaction produced by the man which is 42N, but I am really stuck on question 'b' ... the answer in my book says the center of mass is 2.29m from A, I have drawn a diagram, can anybody please check it and let me know what's wrong with it? also my moments equation is.. [tex] 56N \times 1m + 42N \times 4m = 98N (xm3m) [/tex] http://www.mathhelpforum.com/mathhe...puntitled.jpg 


#2
Dec808, 04:30 PM

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That's fine, except I don't understand where the (x3) comes from … it should be … ? 


#3
Dec808, 04:31 PM

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P: 12,071

I can't see your diagram yet, but your description of the problem seems clear enough to respond.
What is "x" in your moment equation? You seem to be taking moments with respect to end A, which is good. It would make sense to let "x" simply be the distance from A to the centerofgravity position. But you seem to be defining x as something else. 


#4
Dec808, 04:36 PM

P: 112

Momentscalculate centre of mass
but if my equation is correct i still dont get the right answer? 


#5
Dec808, 04:36 PM

P: 112

should it be x4 instead? Cause theres 1 meter in between x and A



#6
Dec808, 04:42 PM

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x is … ? 


#7
Dec808, 04:43 PM

P: 112




#8
Dec808, 04:45 PM

P: 112

thank you! 


#10
Dec808, 04:48 PM

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P: 12,071

Since torque = force times distance, it really is pretty simple. 


#11
Dec808, 04:51 PM

P: 112




#12
Dec808, 04:57 PM

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#13
Dec808, 05:08 PM

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between what two points do you think there is a distance of x3? General hint: whenever you draw a diagram, give letters to all the points on it, so that you can talk about them later … in this case, call the woman's position W, and the centre of mass C … and then decide what two points you want to measure the distance between. 


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