Simple, resultant force problem but stuck.

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Discussion Overview

The discussion revolves around a homework problem involving the determination of the height at which the resultant of three forces acts on a beam. Participants explore the application of static equilibrium principles, particularly focusing on the use of moments to solve for the unknown height.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant calculates the resultant force in the x-direction as Rx=100 lb but expresses uncertainty about its significance and the role of the beam's diameter.
  • Another participant suggests using moments to find the height, recommending the total moments be calculated about a defined point (base B) and using the moment formula (M=distance x force).
  • A later reply indicates that the resultant force of 100 lb acts at a height of 45 inches above base B, questioning whether this height represents the force's location or a relationship between the forces.
  • Another participant confirms that the resultant force is indeed 100 lb at 45 inches above base B, emphasizing the importance of summing moments as well as forces.
  • One participant notes that moments can be taken about any point, prompting a question about the relevance of this flexibility.

Areas of Agreement / Disagreement

Participants generally agree on the approach of using moments to solve the problem, but there is some uncertainty regarding the interpretation of the resultant force's location and its implications. The discussion does not reach a consensus on the final answer or the interpretation of the results.

Contextual Notes

There are unresolved assumptions regarding the definitions of forces and moments, as well as the specific conditions of the problem that may affect the calculations.

frozenguy
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Homework Statement


Determine the height h above the base B at which the resultant of the three forces acts.
prob280.jpg


Homework Equations


Rx=F1,x + F2,x + F3,x

The Attempt at a Solution


Rx=(-300)+650+(-250)
Rx=100

but where?! The picture shows the diameter decreasing a tad just under the 650 force, does that matter?

Anyways, am I suppose to use moments/couples? Dont moments/couples have to be equal in magnitude, opposite/parallel?

Thanks,

Frozenguy
 
Last edited:
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Hi there, Now I remember only a little bit from my statics class, but you did find fx correctly. Now for the height. Moments are the correct way to do it, I would suggest to find the total for all moments and see where you can go from there. Once you get that, think of the moment formula (M=distance x force). And keep in mind your trying to solve for distance.
 
LP20 said:
Hi there, Now I remember only a little bit from my statics class, but you did find fx correctly. Now for the height. Moments are the correct way to do it, I would suggest to find the total for all moments and see where you can go from there. Once you get that, think of the moment formula (M=distance x force). And keep in mind your trying to solve for distance.

Oh ok!

So I add up M1, M2, and M3 (defining base B as the point of rotation) and that will be M0 because of that guys law (I'm terrible, I need to look up his name). Then M0 divided by the magnitude of R will be the distance d.
 
It should be. Do you have an answer for it in the book?
I got an answer that seems reasonable. What's yours?
 
I got that h would be equal to 45" above base B..

It's an even problem, which the book lacks answers for :/

I think this answer makes sense.. so is it saying that at 45" above B, there is 100lb of force acting towards the right? Or is this just to gauge the relationship between the forces?Thanks so much for your help btw.
 
That is exactly what it is saying, your resultant force is 100lb above B 45". This moment equals all the other moments acting on the beam from the other forces. Just like summing up the forces, your summing up the moments.

No problem for the help. I've been helped many times on here before, I figured I'd give back to the community.
 
The beauty of this is that you can take moments about ANY point
 
what do you mean any point? are you referring to verignons lawl?
 

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