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Stretching A Wire

10/14/2017 4:41 PM

Is it possible to stretch a wire tight enough that there is zero sag without breaking? Of course the wire is in a horizontal plane.

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#1

Re: Stretching a wire

10/14/2017 4:59 PM

In principle, no.

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#2

Re: Stretching a wire

10/14/2017 5:11 PM

Only in space beyond most of earth's gravity.

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#3

Re: Stretching a wire

10/14/2017 6:01 PM

Only in zero G or if it has infinite strength. In the real world,on the earth any column, and a wire stretched between 2 points is a thin column, deflects under load, that is calculable from it's physical properties.

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#18
In reply to #3

Re: Stretching a wire

10/16/2017 4:21 AM

If g (not G) is zero, horizontal and vertical have no meaning.

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#4

Re: Stretching a wire

10/14/2017 6:01 PM

..."Carbon fibre has by far the lowest gravitational deflection. A 100 meter long fibre sags only 5.8 mm at breaking tension. Most carbon threads are composed of ~1000 small filaments, 7 microns in diameter"...

https://www.slac.stanford.edu/pubs/slacpubs/11250/slac-pub-11465.pdf

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#5

Re: Stretching a wire

10/14/2017 6:22 PM

.. hello again...

Mounted to spring's that will keep the tension continuous, a short (very) length will not sag.

Without applied tension the wire will sag in short order.

If you want to talk about the atomic level then all bets are off.

that's the long and the short of it

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#6

Re: Stretching a wire

10/14/2017 7:01 PM

No.

One way to look at it is by force vectors. For any segment (dx) on the wire, the vector sum of the forces is equal to zero. These three forces are tension left (N), tension right (N+dN), and weight (ρdx).

https://www.spaceagecontrol.com/calccabl.htm

If there is no sag (h) in the wire, tension left and tension right vectors are horizontal, and there is no vertical component to cancel out weight.

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#7

Re: Stretching a wire

10/14/2017 7:55 PM

Define 'Zero sag'.

In other words: to whatever the accuracy is of your measuring equipment, yes. All measurement systems have a fundamental tolerance error. So to within the ability of your tools to meaure the sag, yes, the wire can be stretched until the sag cannot be measured by your tools, and is thus recorded as 'zero sag'.

In a pure theoretical sense, no. Physics theory says there will always be a sag. But in a practical measurement sense, with the proper equipment for stretching, the sag will be within 5 sigma of zero.

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#8

Re: Stretching a wire

10/14/2017 8:04 PM

Thank you for your answers. I pretty much thought it was not possible in principal. My reason for asking was; in submarine hulls, a wire stretched from port to stbd will slack when the hull is subjected to deep sea pressure, so I was wondering how taut a wire could be made.

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#9
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Re: Stretching a wire

10/14/2017 9:12 PM

If you could lift that same sub into space the wire, or the hull, would fail.

Totally off toic, but:

All The Wacky Submarine Terms You Never Knew You Wanted To Know

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#10
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Re: Stretching a wire

10/14/2017 10:17 PM

" A submarine’s hull actually compresses due to the water pressure it experiences. To prove this, some crews have tied a string tightly between the bulkheads (walls) while on surface. After diving to a deep depth, the string hangs loosely."

http://usnavymuseum.org/Education_LP0022.asp

Even though the sag could never technically reach zero, the sag will still increase when the submarine is in deep sea pressure (I'm sure "how much" is classified. ) As the length across the hull decreases, the string assumes a catenary shape with more sag. Knowing the before and after sag and hull width, the difference in width could be calculated.

Everything gives a little under pressure, even a submarine hull. Some things give a lot more, like turning a large coffee cup into espresso size:

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#12
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Re: Stretching a wire

10/14/2017 11:18 PM
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#20
In reply to #8

Re: Stretching a wire

10/16/2017 9:58 AM

Obviously, not taut enough to not "sag". The degree of "sag" is the matter of some speculation.

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#23
In reply to #8

Re: Stretching a wire

10/16/2017 11:11 AM

You can only "tension" the wire to it's breaking point which you can estimate from it's tensile strength based on what material it is.

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#11

Re: Stretching a wire

10/14/2017 11:03 PM

While I am NOT an engineer, I look at the threads and think about all the answers. (serious ones) the one that catches my attention is the travel at light speed. From what I have read, light speed would take more energy than is possible to achieve. I think the same principle would apply in this case. The tiny amount of energy to straighten the wire in the first instance, would become near infinite when trying to get the wire actually straight. This from a non technical person....

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#15
In reply to #11

Re: Stretching a wire

10/15/2017 10:15 AM

Until the energy required to keep it straight exceeds the materials strength compromises the wires physical properties.

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#13

Re: Stretching A Wire

10/15/2017 7:32 AM

That question comes up in some physics experiments. The longest straightest conductive wire I am aware of is high tensile 25.4 micron diameter tungsten wire. Possibly spider silk wire might be better, but it's not conductive.

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#14

Re: Stretching A Wire

10/15/2017 10:12 AM

No, there will always be Cantenary Sag.

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#21
In reply to #14

Re: Stretching A Wire

10/16/2017 10:04 AM

And if you really want to go into the weeds, research how overhead wire transit train systems use catenary wire sag to keep the conducting wire ALMOST level.

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#24
In reply to #21

Re: Stretching A Wire

10/16/2017 9:41 PM

You take physics and make it work for you.

I design belt conveyors with out belt tightened using catenary sag.

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#16

Re: Stretching A Wire

10/15/2017 1:03 PM

I don't understand why nobody mentioned the length of the wire. The longer the wire, the more weight the ends have to support. If you lessen the length so that the weight is less than the tensile strength of the wire, then it should be able to be done. Why not? However, I understand that spaghetti will probably always have a sag.

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#17

Re: Stretching A Wire

10/15/2017 3:44 PM

No, Mildred, as you would need infinite tension in a gravitational field, and the world doesn't have any materials that will withstand infinite tension.

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#19

Re: Stretching A Wire

10/16/2017 6:56 AM

Dear Mr. ronseto,

Zero sag is impossible while stretching.

One study shows sag up to 7% of span, the pulling force is more or less linear.

When the sag reduces below the 7% level, thee pull force increases, no longer in a linear manner, but exponentialy pull force increases, ultimately the material may break before reaching zero sag.

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#22

Re: Stretching A Wire

10/16/2017 10:50 AM

A wire with a lower density than the medium through which it passes can exhibit a net negative sag. A wire less than three atoms long cannot sag because there is no material in the center(an atom somewhere between and excluding the first and last atom) of the wire. Every wire of length greater than two atoms with a temperature above absolute zero will have thermal oscillations transverse to its length which might be included in a careful definition of "sag". Generally, there is no object longer than two atoms which can ever be construed as perfectly straight. All atoms are subject to Heisenberg uncertainty of location so at Planck order precisions "straight" is undefined, or at the very least uncertain, for wires longer than two atoms. Zero anything(parametric such as the length of a sag), in general, is largely unachievable in real physical materials until quantum effects dominate, at which point the whole discussion changes.

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#25

Re: Stretching A Wire

10/16/2017 9:47 PM

Of course, if one places the wire in a zero gravity field, one should be able to remove all sag.

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#26

Re: Stretching A Wire

10/16/2017 10:25 PM

There are a couple ways to achieve zero sag of a horizontally stretched wire in a gravitational field.

One option is to have the stretched wire falling. If in free fall in a vacuum there can be no sag. In atmosphere, unless in an accompanying downdraft, zero sag could be accomplished with the right design and low turbulence. Alternately zero sag could be achieved without vertical displacement using a steady uniform updraft.

Another option is to have the wire neutrally buoyant. If your wire is the same density as sea water (hollow wire is still a wire) then you can achieve zero sag under sea water. Similarly if your large hollow helium filled wire is neutrally buoyant in air you can achieve zero sag.

A third option is available if you only require zero sag momentarily...vibrate the wire.

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#27
In reply to #26

Re: Stretching A Wire

10/16/2017 11:20 PM

I think you have "compromised" the truth a little.

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#28
In reply to #27

Re: Stretching A Wire

10/17/2017 9:12 AM

Truly? Can you be specific?

If you can detail exactly where you believe I have gone astray, I will either be very appreciative that you have alerted me to the error of my ways, or alternately I will be happy to help you understand what (I believe) you don't yet.

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#29
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Re: Stretching A Wire

10/17/2017 9:53 AM

Hollow wire is either a pipe or a cylinder, but if you wish to model it as a neutral buoyancy wire, who would want to argue with you? Not me.

Suppose you have a happy little wire stretching for hundreds of miles laser-straight in nano-gravity and cannot detect the (obviously existing sag in it) curvature.

What happens when a gravitational wave crosses the plane containing the wire (normal to the nano-gravity)?

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#30
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Re: Stretching A Wire

10/17/2017 12:30 PM

Hollow bar is stiffer than solid shafting...

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#31
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Re: Stretching A Wire

10/17/2017 12:37 PM

..relevance?

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#32
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Re: Stretching A Wire

10/17/2017 3:12 PM

No, opportunity

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#33
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Re: Stretching A Wire

10/17/2017 4:34 PM

very phunny, big guy!

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#34
In reply to #29

Re: Stretching A Wire

10/19/2017 7:05 AM

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#35
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Re: Stretching A Wire

10/19/2017 10:48 AM

Where did you find that?

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#36
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Re: Stretching A Wire

10/19/2017 11:07 AM

http://www.swwireco.com/hollow-conductor-tubing.html

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#37
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Re: Stretching A Wire

10/19/2017 11:13 AM

Thanks.

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