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Maths help two :)

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

  1. lunar

    Gangster

    Joined: Jul 18, 2015

    Posts: 300

    Location: London

    The Earth's radius is about 6400 km.
    Your equation above is what I thought it would be too. However, I was reluctant to respond because I think there is more to the question than meets the eye.
    I recall a similar question been asked in Dr Kharl's radio 5 Science phone-in.
     
  2. esmozz

    Gangster

    Joined: Oct 24, 2013

    Posts: 106

    Technically he's asking about the great circle on a spherical earth which corresponds to the equator. If the earth is not spherical then the calculation may be different.

    The WGS84 datum is reference oblate spheroid used to define the earths surface. It was my understanding that using this datum the earth is defined to approximate a ruby ball, making calculation different. However having just checked it again I may well be wrong, in which case you can ignore everything I've written previously.

    Also, since I'm going to be awkward, the answer is 1m (give or take). I got this by drawing a slice of pie and marking the two radii, the earths radius is so huge compared to the difference between the two radii that the end approximates a box 1m square. Hence 1m. I have no idea if this is a good answer.

    Edited to add: Having thought about it a bit more, I think it's a rubbish answer. I'm going to give up now.
     
  3. h4rm0ny

    Soldato

    Joined: Jun 25, 2011

    Posts: 5,451

    Location: Yorkshire and proud of it!

    I think they meant floating magically above the first string at the same latitude, rather than lying on the ground slighty North of the first one.

    I'd be fascinated to know how you could do that, as the question is "how much longer". It's asking for an absolute value (e.g. 400km) so we need an absolute value to insert into the formula. I.e. the diameter of the Earth.

    Yes, it's an oblate spheroid. But that doesn't make any difference unless we're talking volume. Any given lateral cross section is still going to be a circle (give or take some mountains).
     
  4. Kenai

    Capodecina

    Joined: Apr 5, 2009

    Posts: 17,609

    If the radius increased by 1m, the circumference increased by 6.28m. It doesn't matter whether you started with a basketball, the moon, the earth or the sun. The answer will always be the same.
     
  5. esmozz

    Gangster

    Joined: Oct 24, 2013

    Posts: 106

    Surely if you took a circle round the poles, i.e. normal to the equator you would get an oval. Not that it makes any diffence here.

    So 1m then (give or take):)
     
  6. h4rm0ny

    Soldato

    Joined: Jun 25, 2011

    Posts: 5,451

    Location: Yorkshire and proud of it!

    Someone can shoot me now, please. I wont object.

    Equator was specified. That is a measurement at a specific latitude.
     
  7. FoxEye

    Capodecina

    Joined: Feb 17, 2006

    Posts: 19,531

    Location: Cornwall

    So what's the answer then?

    Not that I care. Maths sucks donkey balls.
     
  8. Kenai

    Capodecina

    Joined: Apr 5, 2009

    Posts: 17,609

    I think you're pushing it a bit to say that 6.28 is 'give or take' the same as 1 :p
     
  9. Crpwned

    Hitman

    Joined: Feb 6, 2013

    Posts: 601

  10. esmozz

    Gangster

    Joined: Oct 24, 2013

    Posts: 106

    I dunno, over 6400,000m its not bad.

    Hard to argue against that so i won't.

    I think the point I was originally trying to make was (and I've had to go away and check since it was 20 odd years since I studied this, so I may still be wrong) that the surface of the earth can be represented mathematically by a geoid based on the WGS84 datum. That geoid is an irregular surface, nonetheless, at the equator you could approximate the circumference using a mathematical series (up to n = as high as you like) - it would not be a circle.

    However all I seem to have proved is I completely missed the point of the original question. Also:

    Me too please.
     
  11. LuckyBenski

    Wise Guy

    Joined: Dec 28, 2017

    Posts: 2,474

    Location: London

    This thread made me feel really smart until I failed to account for the 1m applying to both sides of the earth, and so the answer is 2π and not just π :o
     
  12. lunar

    Gangster

    Joined: Jul 18, 2015

    Posts: 300

    Location: London

    Ironic, where would we be without Maths (and Physics)?
    No Internet, no TV, no mobile phones, no satellites, no radio, no mains electricity.......
     
  13. Angilion

    Man of Honour

    Joined: Dec 5, 2003

    Posts: 15,737

    Location: Just to the left of my PC

    It's also only just an oblate spheroid. The question treats the surface of the Earth as being smooth. Treating Earth as a sphere is an approximation in the same ballpark as that.

    I have to admit that my initial thought about the answer was that the outer rope would be much longer than the inner one, hundreds of kilometres longer. I felt a bit silly when I read the correct answer and realised that yes, of course that was the correct answer. I've known the formula for the circumference of a circle for a very long time and should have gone straight to the correct answer.
     
  14. D3K

    Mobster

    Joined: Nov 13, 2014

    Posts: 2,871

    Post #10 on the front page
     
  15. FoxEye

    Capodecina

    Joined: Feb 17, 2006

    Posts: 19,531

    Location: Cornwall

    Subjective post was subjective.

    It's hard and I'm not very good at it. Ergo it sucks balls ;)
     
  16. Double07

    Wise Guy

    Joined: Jan 10, 2006

    Posts: 1,048

    BEER
     
  17. dl8860

    Mobster

    Joined: Jul 25, 2010

    Posts: 2,627

    Location: Surrey

    How can you say these words and then go on to talk about oblate spheroids. Are you clever or not?

     
  18. h4rm0ny

    Soldato

    Joined: Jun 25, 2011

    Posts: 5,451

    Location: Yorkshire and proud of it!

    I am usually very clever but occasionally can make statements without thinking things through and which are cripplingly embarrassing sometimes.

    Example above.
     
  19. Illgresi

    Hitman

    Joined: Feb 18, 2013

    Posts: 969

    Location: Perth

    1/r times bigger

    or

    100/r expressed as a percentage.