Saturday, January 7, 2017

Rope Around the Earth

Suppose you tie a rope tightly around the Earth at the equator. (Assume the Earth is perfectly spherical, and that the surface is smooth so that the rope lies tight against the surface at all points.) Now suppose that you add an additional 6 feet to the length of the rope. How high off the surface would the rope lie? You could look up the Earth's circumference and do the math to come up with an exact answer, but can you quickly come up with an intuitive guess? (High enough to slide a piece of paper under? To wave your hand under? To walk under?) Click below to see a hint or the answer.


Dan said...

I've got to admit -- as you probably expected -- my intuition was way off.

I love puzzles / illustrations like this. Also is consistent with what we say in software when it comes to performance / optimization -- don't rely on intuition, you're probably wrong... measure, measure, measure!

Keep 'em coming Bill.

Tim Kington said...

I've always loved that one.

Bill the Lizard said...


This is one of my favorites, too. The answer seemed so counter-intuitive until I worked it out, then it seemed like it should have been intuitively obvious.

rajat said...

I saw this puzzle on Discovery channel's show "Mind Games"

Bill the Lizard said...


I've been meaning to watch that show. If it has a lot of puzzles like this one, I'll have to move it up in my "to watch" list.