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Apr 12, 2017
01:07 PM

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Apr 12, 2017
01:07 PM

One task with 3 bobs - Forces searching

I try to find forces in this task. The Mathcad 15 sheet is in attach. Are equations in the Given-Find block right?

What is the 8-th equation?

Solved! Go to Solution.

1 ACCEPTED SOLUTION

Accepted Solutions

May 03, 2017
03:57 AM

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103 REPLIES 103

Apr 12, 2017
01:17 PM

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Apr 12, 2017
01:17 PM

The horizontal forces at the walls don't have to be equal, and

the force vector has to align with the string

Apr 12, 2017
01:44 PM

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Apr 12, 2017
01:44 PM

The horizontal forces at the walls don't have to be equal

If they're not equal, then the system is not static. Those two forces are the only horizontal forces acting on the system, so they must cancel out. Note that your solution has the two forces very close to each other. Remove the negative from your RHS (Valery has defined F0x as acting in the opposite direction as F4x) and re-enable evaluation of that region and your final result is pretty much unchanged.

Apr 12, 2017
02:04 PM

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Apr 12, 2017
02:04 PM

Thanks, Fred.

I think it is the solution: 4 eques and 4 unknowns:

Apr 12, 2017
02:35 PM

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Apr 12, 2017
02:35 PM

You have a solve block, and a matrix solution with the same equations; of course they have the same results!

Attached is a vector force summation at each of the three weights.

I continue to insist that the horizontal forces at the walls do not have to be equal.

Apr 12, 2017
02:51 PM

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Apr 12, 2017
02:51 PM

Sorry, Fred! You have 8 unknown and 7 equations - change guess value an have new result!

We need one more equation!

But we can find F12 and F23 forces not in the solve block!

Apr 12, 2017
03:11 PM

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Apr 12, 2017
03:11 PM

Nope! Sorry, changing the guess doesn't change the answers:

Look at the vector solve block!!

Apr 19, 2017
12:19 AM

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Apr 19, 2017
12:19 AM

Fred Kohlhepp написал(а):

You have a solve block, and a matrix solution with the same equations; of course they have the same results!

The matrix solution does not need the guess values! And same result of course.

May 30, 2017
08:34 PM

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May 30, 2017
08:34 PM

I'm a bit late entering this fray on this point, but I agree with Fred. The net horizontal force is zero only if the center-of-mass(COM) doesn't move horizontally. This is clearly true in the static case, but I doubt the geometry of the strings constrains things in this manner for arbitrary angles of the various strings in a dynamic situation. In this case, there must be a net horizontal force consistent with the horizontal motion of the COM.

Lou

Apr 12, 2017
02:41 PM

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Apr 12, 2017
02:41 PM

Solution without an animation is not solution!

But we must put TOL=CTOL-10^-7 (not 10^-): we work with cm not m - with gm not kg - it is one bug of Mathcad 15!

Apr 12, 2017
03:02 PM

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Apr 12, 2017
03:02 PM

Valery,

The 4th String gets shorter and shorter.

Can this be? I don't belive that.

Volker

Apr 12, 2017
03:08 PM

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Apr 12, 2017
03:08 PM

We do so - Pull and shorten the 4-th string!

Apr 12, 2017
04:01 PM

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Apr 12, 2017
04:01 PM

Valery likes animations. He chose L34 to be the variable that changes over time (FRAME).

Apr 12, 2017
05:37 PM

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Apr 13, 2017
12:11 AM

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Apr 13, 2017
12:11 AM

Wunderbar!

Apr 13, 2017
12:09 AM

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Apr 13, 2017
12:09 AM

I think not only a fine animation but about TOL and CTOL!!! Compare pls this animation and at TOL and CTOL=10^-7 (above)

Apr 13, 2017
12:38 AM

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Apr 13, 2017
03:15 PM

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Apr 13, 2017
03:15 PM

Solution in Prime with compare with values from real experiment -see please the first picture in this discussion.

гс - грамм силы - gram of force

It is picture for the future article and a study for the book. Thanks to all!

Apr 14, 2017
02:51 AM

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Apr 14, 2017
02:51 AM

Apr 14, 2017
04:18 AM

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Apr 14, 2017
04:18 AM

Italia = art!

Apr 14, 2017
10:48 AM

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Apr 14, 2017
10:48 AM

We can use the symbolic solution for forces

can we have it for x and y?

Apr 20, 2017
01:45 AM

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Apr 20, 2017
01:45 AM

We can find F12 and F23 so!

OK?

Apr 13, 2017
03:13 AM

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Apr 13, 2017
03:13 AM

New task - change please Inextensible threads to tensile threads!

Слабо? With use the Hooke's law!

The sheet with Inextensible threads is in the attach!

Apr 13, 2017
09:19 AM

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Apr 13, 2017
09:19 AM

It seems that we have no chance to solve the problem without the "Energy" Method which is a very elegant way to go!

Minimizing the potential Energy is what nature does make.

Thank you for giving me this to understand, Valery.

Trying to solve with Equations in the solve block with guess-Values doesn't sucess. Don't know, why.

Volker

Apr 13, 2017
10:18 AM

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Apr 13, 2017
10:18 AM

Volker Lehner написал(а):

It seems that we have no chance to solve the problem without the "Energy" Method which is a very elegant way to go!

Minimizing the potential Energy is what nature does make.

Thank you for giving me this to understand, Valery.

I am glad to see it!

Volker Lehner написал(а):

Trying to solve with Equations in the solve block with guess-Values doesn't sucess. Don't know, why.

Attach please the sheet!

Apr 13, 2017
10:30 AM

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Apr 13, 2017
10:30 AM

There are two Versions:

The one is about the Energy Method (Index 01) and the other one is the 10 EQN solution which is instabil (means different results when guess values changes)

Volker

Apr 12, 2017
01:33 PM

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Apr 12, 2017
01:33 PM

The reactions at 0 and 4 must be in the direction of the string.

For example:

(negative required due to how your variables are defined)

You can write a similar equation for the forces at point 4.

Also, you can balance the forces in the X direction at points 1, 2, & 3, and you can balance the forces in both the X and Y directions at points 0 & 4. Some of these equations will likely be redundant (what's the mathematical term for this?).

Apr 12, 2017
02:01 PM

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Apr 12, 2017
02:01 PM

The horizontal forces at the **weights** are equal (for static equilibrium.) The walls are fixed, so there is no requirement that F0x and F4x be equal in magnitude.

Yes, I was sloppy in directions so I needed a negative sign.

Apr 12, 2017
02:11 PM

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Apr 12, 2017
02:11 PM

Fred,

I agree the walls are fixed, but look at the string. It has 5 forces acting on it. 3 of those forces are vertical (points 1, 2, and 3), so the horizontal components of the other two (0 & 4) must be equal and opposite.

Apr 12, 2017
02:48 PM

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Apr 12, 2017
02:48 PM

Hi Valery Ochkov,

This is the well known case of Funicular Polygon, extensively studied in rational mechanics.