The Other Other Rope Around The Earth

Recent changes
Table of contents
Links to this page


My latest posts can be found here:
Previous blog posts:
Additionally, some earlier writings:

The Other, Other Rope Around The Earth Problem - 2015/08/01

There's a classic problem:

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:

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 Puzzle World Vs Crypto World 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 <<<<
Photocopy A Mirror
>>>> Next >>>>
A Mirror Copied ...

You should follow me on twitter @ColinTheMathmo


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.



Links on this page

Site hosted by Colin and Rachel Wright:
  • Maths, Design, Juggling, Computing,
  • Embroidery, Proof-reading,
  • and other clever stuff.

Suggest a change ( <-- What does this mean?) / Send me email
Front Page / All pages by date / Site overview / Top of page

Universally Browser Friendly     Quotation from
Tim Berners-Lee
    Valid HTML 3.2!