# The Naked Scientists Forum

### Author Topic: String around the earth!!  (Read 11952 times)

#### woodyapex

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• Posts: 7
##### String around the earth!!
« on: 27/03/2008 00:08:08 »
Pretending the earth is perfectly smooth! You wrap a piece of string around it !!
Pull it nice and tight so both ends meet!!
Then add 1 m of string to the other piece , so in effect you have the circumference of the earth + one meter .
Now pretend for a second you can reverse the gravitational pull and levitate the string so it is parallel!!
How far off the earth would the gap be ??
« Last Edit: 27/03/2008 08:27:26 by woodyapex »

#### DoctorBeaver

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• A stitch in time would have confused Einstein.
##### Re: String around the earth!!
« Reply #1 on: 27/03/2008 07:56:06 »
Sorry, I can't help with the question; but are you thinking of entering a Red Bull Flugtag?

Also, could you please modify the title to reflect the question you are asking. We like that sort of thing here and it will help with getting replies.

#### BenV

• Neilep Level Member
• Posts: 1503
##### String around the earth!!
« Reply #2 on: 27/03/2008 10:54:49 »
We should be able to work this out fairly simply.  These figures are not exact, but the maths should hold true.  As we're just looking at a 'slice' around the earth, we can treat this as a normal circle.

Assuming the circumference of the Earth at the equator to be 40,075,020m (figure from wikipedia - it'll do for this purpose, surely?)

And knowing that Circumference/π = diameter

we can see that 40,075,020/π = 12,756,275.1

If we then add 1 meter to our circumference

we get 40,075,021/π = 12756275.4

So if the string (and therefore circumference) is 1 meter longer, it would give a diameter 0.3m wider.  If the string was hovering, this extra would be shared on either side of the earth, and therefore would be 0.15m, a mere 15 cm, off the ground.

That's the long winded way of going about it, but as Daveshorts has just pointed out to me it's easier to say that the circumference is 2π times the radius, so if you increase the circumference by 1m you will increase the radius by 1m/(2π), which just happens to be 0.16m.

#### graham.d

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##### String around the earth!!
« Reply #3 on: 27/03/2008 12:00:11 »
Circumference(C) = 2πR

Increase by 1m =>

C+1 = 2π(R+x) where x is the increase in radius (how much it would be above the ground)

so 2πR+1 = 2π(R+x)

x = 1/(2π) = 15.9154943 cm (approx)

No need to know the size of the earth

#### BenV

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• Posts: 1503
##### String around the earth!!
« Reply #4 on: 27/03/2008 12:52:22 »
Seems so simple, doesn't it?  I just launched into the numbers without thinking about how to simplify.  I hope there's a lesson in there for someone!

#### Bored chemist

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##### String around the earth!!
« Reply #5 on: 27/03/2008 19:54:05 »
It's odd to think that it dosn't matter how big the earth is, the answer comes out the same.
If the earth were practically non existent dot you would have a circle with 1 metre circumference and therefor a radius of about 6 inches.

#### DoctorBeaver

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##### String around the earth!!
« Reply #6 on: 27/03/2008 20:04:41 »
OY! You didn't just alter the title, you altered the whole goddam post! Now you've made me look stupid by answering a question you didn't ask  [:(!]

#### woodyapex

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##### String around the earth!!
« Reply #7 on: 27/03/2008 22:08:17 »
OY! You didn't just alter the title, you altered the whole goddam post! Now you've made me look stupid by answering a question you didn't ask  [:(!]
sorry doctor beaver .
yes guys doctor beaver could not answer the other question i posed . original question to follow , with correct title mr Beaver ;)

#### woodyapex

• First timers
• Posts: 7
##### String around the earth!!
« Reply #8 on: 27/03/2008 22:10:30 »
Circumference(C) = 2πR

Increase by 1m =>

C+1 = 2π(R+x) where x is the increase in radius (how much it would be above the ground)

so 2πR+1 = 2π(R+x)

x = 1/(2π) = 15.9154943 cm (approx)

No need to know the size of the earth
your right graham , but ask most people they say hardly a mm .

#### woodyapex

• First timers
• Posts: 7
##### String around the earth!!
« Reply #9 on: 27/03/2008 22:11:31 »
I've got another question i need help with that i don 't know the answer to please, take a look at it for me.

#### The Naked Scientists Forum

##### String around the earth!!
« Reply #9 on: 27/03/2008 22:11:31 »

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