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File: TheOtherOtherRopeAroundTheEarth ''' <link rel="alternate" type="application/rss+xml" ''' href="/rss.xml" title="RSS Feed"> ********> width="25%" |>> ''' <a title="Subscribe to my feed" ''' rel="alternate" ''' href="https://www.solipsys.co.uk/rss.xml"> ''' <img style="border-width: 0px;" ''' src="https://www.feedburner.com/fb/images/pub/feed-icon32x32.png" ''' align="middle" ''' alt="" />Subscribe!</a> _ ''' <a href="https://twitter.com/ColinTheMathmo"> ''' <img src="https://www.solipsys.co.uk/new/images/TwitterButton.png" ''' title="By: TwitterButtons.net" ''' width="212" height="69" ''' alt="@ColinTheMathmo" ''' /></a> <<| ---- My latest posts can be found here: * ColinsBlog ---- Previous blog posts: * PhotocopyAMirror * ThePointOfTheBanachTarskiTheorem * SieveOfEratosthenesInPython * FastPerrinTest * RussianPeasantMultiplication * FindingPerrinPseudoPrimes_Part2 * FindingPerrinPseudoPrimes_Part1 * TheUnwiseUpdate * MilesPerGallon * TrackingAnItemOnHackerNews * HackerNewsUserAges * PokingTheDustyCorners * ThereIsNoTimeForThis * PublicallySharingLinks * LearningTimesTables * GracefulDegradation * DiagrammingMathsTopics * OnTheRack * SquareRootByLongDivision * BeyondTheBoundary * FillInTheGaps * SoftwareChecklist * NASASpaceCrews * TheBirthdayParadox * TheTrapeziumConundrum * RevisitingTheAnt * TheAntAndTheRubberBand * IrrationalsExist * MultipleChoiceProbabilityPuzzle * RandomEratosthenes * WrappingUpSquareDissection * DissectingASquarePart2 * DissectingACircle * DissectingASquare * AnOddityInTennis * DecisionTreeForTennis * DecisionTreesInGames * AMatterOfConvention * DoYouNourishOrTarnish * BinarySearchReconsidered * TwoEqualsFour * TheLostPropertyOffice * TheForgivingUserInterface * SettingUpRSS * WithdrawingFromHackerNews ---- Additionally, some earlier writings: * RandomWritings. * ColinsBlog2010 * ColinsBlog2009 * ColinsBlog2008 * ColinsBlog2007 * ColinsBlogBefore2007 ******** !!! The /Other,/ Other Rope Around The Earth Problem - 2015/08/01 There's a classic problem: [[[>30 If you don't know this one you really need to have a go at it. ]]] * Upon stretching a rope around the Earth, you find that you have 6 metres excess. So you join the ends, and then go around the Earth propping up the rope equally everywhere. How high will it be? An alternative that's been suggested is this: [[[>30 This is also a fun question, although less elementary. ]]] * Instead of propping it up equally everywhere, just prop it up as high as possible in one place. If it doesn't sag, how high is the prop? But now Bill Mullins has asked me yet another variant: * Suppose you have a rope, propped up one metre above the Earth's surface, running right around the Earth. Now suppose instead of being rope, it's steel. Release the props. _ _ What happens? If it falls, where? Assuming it remains stiff (and we're in PuzzleWorldVsCryptoWorld so let's assume it does), that means that if it falls in one place, that means that on the other side of the world it rises. Maybe it just floats there, stable. Or maybe it does fall in one place, rises at the antipodal point, and then remains there, off-centre, grazing the Earth's surface. What happens? If we consider the centres of mass of the Earth and the ring of steel, we can consider what happens when the ring is off centre. The two centres of mass will surely be attracted, so surely the ring floats back to be centred, doesn't it? Or does it? We know that we can't always replace objects with a point mass at their centre of mass, and maybe this is one of those cases. So what /does/ happen? ---- |>> | |>> <<<< Prev <<<< ---- PhotocopyAMirror <<| | : | |>> >>>> Next >>>> ---- AMirrorCopied ... <<| | ---- ********> ''' <a href="https://twitter.com/ColinTheMathmo">You should follow me on twitter</a> ******** ''' <a href="https://twitter.com/ColinTheMathmo"> ''' <img src="https://www.solipsys.co.uk/new/images/TwitterButton.png" ''' title="By: TwitterButtons.net" ''' width="212" height="69" ''' alt="@ColinTheMathmo" ''' /></a> ********< <<| ---- !! Comments I've decided no longer to include comments directly via the Disqus (or any other) system. Instead, I'd be more than delighted to get emails from people who wish to make comments or engage in discussion. Comments will then be integrated into the page as and when they are appropriate. If the number of emails/comments gets too large to handle then I might return to a semi-automated system. That's looking increasingly unlikely. ********<