# Maths help two :)

Discussion in 'General Discussion' started by Orionaut, Apr 21, 2019.

1. Orionaut

# Posts: 6,522

The other thread just reminded me of a question I saw many years ago (In New Scientist IIRC)

Consider the situation...

Imagine a string wrapped all the way around the Earths equator.

Now imagine a second string raised to a point 1M above the earths equator.

Q: How much longer is the second string than the first.

I am not searching for the answer here, simply the method.

What was your solution, and how did you arrive at it?

2. Jasoncmor

# Posts: 419

The second would be shorter, would it not?

3. D3K

# Posts: 2,880

(2*pie*[r+1]) - (2*pie*r)

Where r is the average radius of the earth in metres at the equator.

But if you want to lay the string flat against every nook and cranny of the earth to get a true measurement, good luck

4. Orionaut

# Posts: 6,522

Correct, but too complicated.

There is a simpler way of looking at it

Thats what I am looking for, not the solution but rather the logic.

5. touch

# Location: Sufferlandria

The equator doesn't have any altitude. There's no such thing as "1 metre above the equator"?
Is it 1m above sea level at the equator or 1m above ground level? They're going to be very different things.

6. Kenai

# Posts: 17,968

It's a problem to demonstrate that to calculate the change in circumference of a circle as a result of change in diameter/radius doesn't require you to know the original circumference or diameter/radius.

You're looking at it too literally if you're worrying about sea level vs ground level.

7. D3K

# Posts: 2,880

You want me to factorise it?

2*pie*(r+1/r)

You need to know the radius to calculate the specific change. Even if you don't know it, radius/diam is still required in the equation.

8. Kenai

# Posts: 17,968

If you want to demonstrate working from first principle maybe but once you know that all the original figures drop out, it means you can happily calculate the change in one of those numbers simply by knowing the change in the other.

9. Orionaut

# Posts: 6,522

All the information required is in the question.

10. D3K

# Posts: 2,880

Yeah ok fml it's 2 pie... meh

11. Kenai

# Posts: 17,968

I was trying to post in a way that didn't just kill the thread half a dozen posts in

12. builder22

# Posts: 2,565

is this second string just hovering in mid air?

are drones/helicopters/spaceship allowed to help???

13. TNGL

# Posts: 2,473

Classic job interview brain teaser.

14. Orionaut

# Posts: 6,522

True

That was my logic when I first saw it. I haven't been told what the earths radius is, therefore it doesn't matter

Once you accept that, the answer is clear.

It is a bit like the "Monty hall" problem.

People feel the need to come up with all sorts of complex arguments to explain how this, apparently, counter intuitive situation can arise.

In fact it isn't complex at all.

The solution is that it has little to do with probability and everything to do with the fact that the game show host knows which doors have the car and which have the Goat!

15. builder22

# Posts: 2,565

btw, when you said 'raised to a point 1m above the equator' I thought you meant towards the North Pole, with the second string then being smaller, still wrapped round the earth....missed the fact you said the second string would be bigger!

16. viathinair

17. Pringle W

# Location: Glebe Park

With all the winky faces by the op is the answer something humourous to do with eating pies?

18. esmozz

# Posts: 182

The world is'nt a sphere.

Google WGS84 and then work it out. Personnally I would have no idea.

19. D3K