Calculating Force Needed to Pull Bungee Taught

[bob012345]

Using the original numbers of 9 lbs. per 1000 ft, converting to metric units, I get that a one mile cord (1.6km) sags 12m with the tension equal to 825 lbs or 3670N. To get the sag down to 0.12m or about 5 inches the tension would rise 100 fold to about 37 metric tonnes. Of course it would break long before that. Including the 100% stretch might halve that but I think it's still way beyond the bungee cord specs.My conclusion is that you will have to find another way to create the illusion of framing these sculptures. I like @DaveC426913's idea of using laser light. It reminds me of this artist using laser beams to make 'intergalactic sculpture'. You could literally use smoke and mirrors.

http://maarav.org.il/english/2018/04/17/intergalactic-sculpture-1986-2001/

 

[Vanadium 50]

It would really help if you were to stop messing with us and tell us what the problem you are trying to solve is and not make us guess.

You should also take a look at telephone or electrical wires. Ever see them strung between poles a mile apart? There's a reason for that.

 

[fascinated]

You guys are great. I have been googling away, learning about lasers a bit, and don't understand how the beam would be visible over that distance? Wouldn't it just project a dot on the other side, wherever it hits?

And a video of this guy shooting a laser over a lake popped up. He claims the laser was four feet above the water and his cell phone picked it up from just inches above the water on the other side.

Measuring curvature of the earth.
Over a one nautical mile span, the Earth curves by one minute of arc (1/60th of one degree).

What would it be over that distance? He claims 16.4 miles.

Is this a trick?

It would really help if you were to stop messing with us and tell us what the problem you are trying to solve is and not make us guess.

You should also take a look at telephone or electrical wires. Ever see them strung between poles a mile apart? There's a reason for that.

I'm not messing with anyone. I'm just curious and not super smart, and I appreciate everyone who takes the time to explain things that must seem so obvious to them.

And no, I have never seen that, but when I googled it, I found this:
The Ameralik Span is the longest span of an electrical overhead powerline in the world. It is situated near Nuuk on Greenland and crosses Ameralik fjord with a span width of 5.37 km (3.34 miles).
https://www.guinnessworldrecords.com/world-records/longest-span-of-an-electrical-overhead-powerline

 

[bob012345]

And a video of this guy shooting a laser over a lake popped up. He claims the laser was four feet above the water and his cell phone picked it up from just inches above the water on the other side.

How high does your frame need to be? Could you provide a better link to that video?

 

[fascinated]

How high does your frame need to be? Could you provide a better link to that video?

10 feet would be perfect.

And this might get close. Spectra braid line rated for 78 pounds with specific gravity of .98. And 2000 meters weighs about .37 lbs.

 

[bob012345]

10 feet would be perfect.

And this might get close. Spectra braid line rated for 78 pounds with specific gravity of .98. And 2000 meters weighs about .37 lbs.

0.37 lbs. for what? The whole 2000 meters?

 

[bob012345]

Here is a cable sag calculator;

https://www.spaceagecontrol.com/calccabm.htm?F=351&a=1625&q=0.0000841&g=9.81&Submit+Button=Calculate

 

[fascinated]

Could you provide a better link to that video?

Timestamped at the description:

0.37 lbs. for what? The whole 2000 meters?

Yes, if my math is correct.

 

[bob012345]

Yes, if my math is correct.

Interesting but misleading. The world is not flat. I hope your project is not related to that idea.

https://flatearth.ws/laser

 

[fascinated]

Interesting but misleading. The world is not flat. I hope your project is not related to that idea.

LOL, at flat. There must be some reasonable explanation. I am more curious about using a laser with the beam being visible as suggested. Catenary handled.

 

[bob012345]

LOL, at flat. There must be some reasonable explanation. I am more curious about using a laser with the beam being visible as suggested. Catenary handled.

If we are talking about only one mile it is easier. The beam diverges a lot over 16 miles and is not focused at a point across the lake. Your line above would still sag 2.5 feet over one mile. Try using that calculator I referenced.

 

[fascinated]

If we are talking about only one mile it is easier. The beam diverges a lot over 16 miles and is not focused at a point across the lake. Your line above would still sag 2.5 feet over one mile. Try using that calculator I referenced.

I actually came across that same calculator, but didn't understand how to enter the right numbers.

And you are saying that line I found above would sag 2.5 feet or that the laser will curve that much?

 

[jbriggs444]

What would it be over that distance? He claims 16.4 miles.

Is this a trick?

16.4 [nautical] miles = 16.4 minutes of arc. Likely we are dealing with refraction. A laser beam in air need not be straight.

But let us do the math. Over a 16.4 mile span, how much higher does the curvature of the Earth make the midpoint than the ends? For convenience, let's call it 1/4 of a degree.

Think of the lake as an arc of a circle centered directly above the Earth's center. From there it extends left and right by 8.2 miles, curving slightly downward both ways.

The lake's midpoint should have a height above the center of the Earth of 6000 miles times cosine of zero degrees. The endpoints should each have a height of 6000 miles times the cosine of 1/8 degree.

The cosine of 1/8 of a degree is 0.9999976. Which is 0.0000024 less than 1. Multiply 6000 miles by this and you get 75 feet. So yes if there were no refraction and no diffraction and both sender and receiver were near ground level, that laser should have been hard to see behind a 75 foot tall arc of water.

The amount of refraction depends on many factors. Notably humidity and temperature gradient. A cool lake under hot air would tend to produce downward-curving refraction. By contrast, hot pavement and cooler air produces upward-curving refraction [aka mirages].

But we are supposed to be talking about cords, tension and sagging. Not flat Earth conspiracy theories.

 

[bob012345]

I actually came across that same calculator, but didn't understand how to enter the right numbers.

And you are saying that line I found above would sag 2.5 feet or that the laser will curve that much?

If I understood how to enter the data properly, the line sags about 2.5 ft. if it is one mile long and has the tension you wanted. You have to put in how much tension the line can handle, the length and the weight per unit length in consistent units..

 

[Shane Kennedy]

If it is held at anything other than vertical, it will get closer and closer to a straight line, but never achieve it.

 

[bob012345]

You could use an almost invisible thin cord in a catenary to uphold a more visible cord in a straight line like a suspension bridge.

 

[rbelli1]

Any idea for how much sag with piano wire and if these lengths are accessible?

2000 pounds, more or less.

Please don't try this. When this wire breaks it turns into an unpredictable flying Ginsu knife that could maim or kill someone.

BoB

 

[hmmm27]

or that the laser will curve that much?

Laser beams don't curve (except when they do because of refractive atmospheric conditions along the path : think "mirage"... oh, and technically gravity affects light as well as everything else : good luck measuring that).

 

[sophiecentaur]

@fascinated This whole project worries me. You can't go hanging lines over roads and other peoples' property without satisfying loads of regulations, getting way leave and (unless you are mad) getting insurance.

The picture of a rubber band is a complete red herring but you could measure a finite sag in it, using interferometry.

Imo this is a finite safety risk and some of the above posts make this clear, regarding the energy stored. (Did you ever consider how you might install this thing?)

 

[bob012345]

@fascinated This whole project worries me. You can't go hanging lines over roads and other peoples' property without satisfying loads of regulations, getting way leave and (unless you are mad) getting insurance.

The picture of a rubber band is a complete red herring but you could measure a finite sag in it, using interferometry.

Imo this is a finite safety risk and some of the above posts make this clear, regarding the energy stored. (Did you ever consider how you might install this thing?)

I suspect it is some sort of outdoor art exhibition/festival probably in a large field.

 

[sophiecentaur]

some sort of outdoor art exhibition

Haha - right. So it would require "some sort of" qualified Engineer to design a safe version, particularly if the general public would be exposed to it. I think it may be legal to expose oneself to high risks but . . . .

The Energy stored in stretched lines can be dangerously high and the bungier the line, the more Energy is stored. It scares me.

 

[bob012345]

Haha - right. So it would require "some sort of" qualified Engineer to design a safe version, particularly if the general public would be exposed to it. I think it may be legal to expose oneself to high risks but . . . .

The Energy stored in stretched lines can be dangerously high and the bungier the line, the more Energy is stored. It scares me.

I wonder if it might be possible to make the line out of colored paper, the kind of strips for party decorations?

 

[sophiecentaur]

I wonder if it might be possible to make the line out of colored paper, the kind of strips for party decorations?

A short length could self-support but any wind would tear it. Then here's the problem of rain . . . . . .

The best thing (for a much shorter installation) would actually be steel wire. Minimal stretch so less sag and less stored Energy.

 

[DaveC426913]

The Energy stored in stretched lines can be dangerously high and the bungier the line, the more Energy is stored. It scares me.

The more I think about it, the more I concur. There's almost no way to do this that doesn't involve a dangerous amount of stored energy. That should not be understated.

 

[fascinated]

Thanks again for all the replies.

If it is held at anything other than vertical, it will get closer and closer to a straight line, but never achieve it.

Yep, got that.

You could use an almost invisible thin cord in a catenary to uphold a more visible cord in a straight line like a suspension bridge.
View attachment 290058

This! It was brought up before as I couldn't see how this gets done. With the spectra braid line, we have the sag down to 2.5 feet. This would allow us to hang the first line at 12.5 feet and have a perfect line at 10 feet.

My question again is, what can be used for the bottom line? If it is just another "more visible" line, how could it ever form a horizontal line without nearly infinite vertical supports? Or am I totally missing this one too?

Thanks again

Please don't try this. When this wire breaks it turns into an unpredictable flying Ginsu knife that could maim or kill someone.

BoB

Lol... at least when the bungee snaps, it goes almost perfectly straight to where it broke.

The picture of a rubber band is a complete red herring but you could measure a finite sag in it, using interferometry.

I got you on everything else, but this one is still tough. I *almost* think that's true.

I suspect it is some sort of outdoor art exhibition/festival probably in a large field.

Private land owner... but who knows.

 

[fascinated]

Haha - right. So it would require "some sort of" qualified Engineer to design a safe version, particularly if the general public would be exposed to it. I think it may be legal to expose oneself to high risks but . . . .

The Energy stored in stretched lines can be dangerously high and the bungier the line, the more Energy is stored. It scares me.

It's impossible. We have to do it a different way.

The more I think about it, the more I concur. There's almost no way to do this that doesn't involve a dangerous amount of stored energy. That should not be understated.

I think the supported, more visible line would work, if that is possible. I still don't see how we guarantee a horizontal line.

Thanks again, folks.

 

[DaveC426913]

I think the supported, more visible line would work, if that is possible. I still don't see how we guarantee a horizontal line.

It won't. All it will do is make more (granted, shorter less saggy) caternaries.

 

[jbriggs444]

5000 feet of helium-filled tube constructed of aluminized Mylar and deployed on a dead calm day?

 

[pbuk]

High voltage electricity pylons are generally about 1,000 ft apart. You want something 5 times further apart. Forget it.

 

[berkeman]

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