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Force couples 
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#1
Nov305, 01:31 PM

P: 51

hi
If two equal forces in oposite direction act on an object in translational equilibrium it will just rotate and continue to be in translational equilibrium.No matter where on the body two forces are applied? As simple as possible please: How did we come up with torque= F * distance_between_two _forces Can you tell me why formula is correct no matter where the rotation axis is located? What if body has no rotation axis,if it is just your regular book or something like that? Could these two forces actually represent the sum of many forces applied on multiple points on the body,or must there be only two forces? thank you for your time 


#2
Nov305, 02:11 PM

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P: 2,277

Why don't you prove it?
Start with a rigid solid and put two forces with same magnitude but opposite directions, then calculate the moment of each force at a point O and add them. 


#3
Nov405, 03:12 AM

P: 51

prove what?There are 5 questions I asked,and in any case I'm not shure I know what you are talking about



#4
Nov405, 06:45 AM

HW Helper
P: 1,454

Force couples
Use this drawing to "proof" the formula.



#5
Nov505, 01:18 PM

P: 51

I did proof but still have two questions
M=F( r1*sin(a) + r2*sin(b) )=F*d d=F*r1*sin(a) + r2*sin(b) What if body has no rotation axis,if it is just your regular book or something like that? Could these two forces actually represent the sum of many forces applied on multiple points on the body,or must there be only two forces? 


#6
Nov505, 05:43 PM

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P: 2,277




#7
Nov605, 02:31 PM

P: 51




#8
Nov605, 02:47 PM

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P: 2,277

What i was saying is if you consider Varignon's Theorem, and if you have a finite numer of vector forces concurrent to one point and then another set of vector forces concurrent to another point, and if you calculate the resultant force of each of both sets, and the resultant force of each set has the same magnitude, but opposite direction (thus being in the same plane), then you indeed can form a couple out of these two set of forces. 


#9
Nov605, 03:00 PM

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P: 2,277

I was thinking a little more about your question, yes a Couple could be the statically equivalent system for a system of many forces acting in different points, coplanar o not.



#10
Nov805, 11:30 AM

P: 51




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