# Dr Tickle

2

#### 2scoops0406

Here's one for you :-

Assume the earth is perfectly spherical.

You have a rope that stretches around the equator the ends of which just touch each other so given that the earths radius is 6378.137 Km (according to google) this would mean that that the length of the rope would be approximately 40055 Km.

Now, say we want the rope to be suspended uniformly 1 metre above the equator.

Question, how much extra rope do we need to allow for this.

Without using a calculator, what do you immediately think?

Nothing ?
10 metres?
1 Kilometre ?
100 kilometres?

You would need an extra 2*Pi (=2*3.141592653589793238462643383279.......) Metres of rope.

the rope forms a circle, radius x metres, circumference 2*pi*x. The new circle has a radius of 2*pi*(x+1).

(2*pi*(X+1)) - (2*pi*x) = 2*pi

Virtually nothing as 1m is 0.001km, therefore by finding the circumference by adding this small amount to the radius gives very little change - especially if the circumference is given to the nearest km - no change at all.

Does water go down a plughole clockwise in the northern hemisphere and anticlockwise in the southern hemisphere? Or the other way around ? or does it not make any difference ?

Well I had no idea Eddie, so I got the calc out. The rope would be 40075km long to start with not 40055km.
I make it that the rope would have to be an extra 62.8 metres (working from the google figure) so nelsy is right in saying that it's not much difference no?

gcol said:
Well I had no idea Eddie, so I got the calc out. The rope would be 40075km long to start with not 40055km.
I make it that the rope would have to be an extra 62.8 metres (working from the google figure) so nelsy is right in saying that it's not much difference no?

Nearly but not quite right. (Yes 40075km is more accurate, but I just used 3.14 as pi)

Hard to judge as the rope would sink/sag where it touches the oceans so extra length would be added.

Don't start, it's a theoretical question

Eddie M said:
Don't start, it's a theoretical question
Yes, I think it's a bad idea to suspend a rope 1 metre off the ground around the world, it might get tangled around the plane on the conveyor belt and then the damn thing will never take off...

ninebob said:
Eddie M said:
Don't start, it's a theoretical question
Yes, I think it's a bad idea to suspend a rope 1 metre off the ground around the world, it might get tangled around the plane on the conveyor belt and then the damn thing will never take off...
ROFL

It may not take off, but would it rotate clockwise or anticlockwise depending on which hemisphere it's in ?

TicklyT said:
This is the only correct answer on the topic - the rest of them range from wrong to downright beserk.

Softus said:
TicklyT said:
This is the only correct answer on the topic - the rest of them range from wrong to downright beserk.

You've spoiled it, what about water down a plughole then?

Eddie M said:
what about water down a plughole then?
It's different in the different hemispheres, but that has nothing to do with this topic.

Softus said:
Eddie M said:
what about water down a plughole then?
It's different in the different hemispheres, but that has nothing to do with this topic.

No, it's not

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