Answer to Puzzle #41: Rope Around the Earth

41. Suppose you tie a rope tightly around the Earth's equator. You add an extra 3 feet to the length. All around the Earth the rope is raised up uniformly as high as is possible to make it tight again. How high is that?

This puzzle is old, a version of it appears in William Whiston's The Elements of Euclid circa 1702. More latterly it appears in Showtime drama 'House of Lies' about a group of management consultants.

picture of the earth with a rope around it Before reading the answer can I interest you in a clue?

This puzzle is not so much hard as the result is surprising. Even as a I write it I know the answer is correct but it still doesn't sit well. Because of this we are going to do a rigorous mathematical treatment.

Let CE be the circumference of the Earth, LR be the new length of the rope, RE & RR be the radius from the center of the Earth to the edge and the new rope respectively. We are interested in what we will call H = RR - RE. Let's call X the amount we add to the length of the rope, in our case 3 feet. Additionally we know the relationship between the circumference of a circle and it's radius is generally given by R = C/2π that is the only formula you need to know to solve this puzzle. Plus some basic algebra.

equation to solve rope around the earth interview question

So for us the answer is 5.73 inches. In an interview a bit less than 6 would be sufficient.

What Happened?

The key thing to note is that the radius or circumference of the Earth do not feature in the result at all. They dropped out of the equations. In fact the answer would be the same if you tied a rope around your waist and added 3 feet to that. Tough to accept I know. But it's true.

© Nigel Coldwell 2004 -  – The questions on this site may be reproduced without further permission, I do not claim copyright over them. The answers are mine and may not be reproduced without my expressed prior consent. Please enquire using the link at the top of the page.

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