The string is now rearranged so that it is at a uniform height above the equator.
Rope around the earth add 1 meter.
How high is that.
What will its height be above the surface.
Now untie the rope and add an extra 1 meter of rope and make it a perfect circle and place it around the.
Rope around the world you have a piece of rope that just fits around the earth.
In fact this brain teaser requires neither an exact measurement of the earth s circumference which in fact varies by many kilometers depending on which circumference you measure nor even an assumption that the earth has a circular cross sect.
Now let us tie a rope tightly around its equator.
Now imagine lifting off this very long rope don t ask me how cutting it somewhere so as to stitch into it exactly one meter of extra rope.
Then imagine placing this longer rope back around the earth s equator.
The radius of earth is 6371 km approx.
From there it s not hard to believe that adding 3 feet to a rope around the actual earth would raise it almost 6 inches.
The idea is to imagine the earth is a cube or just a square really and ask yourself if you added say 8 feet to the rope how far would that raise it above the square earth.
This string is cut and a piece 1 metre 3 ft 3 in in length is added in.
Imagine a rope tied around the earth s equator like a ring on a person s finger.
Imagine a rope that fits snugly all the way around the earth like a ring on a person s finger.
Now imagine the rope is made just one meter longer and lifted uniformly off the surface until it is once again taught.
From the diagram it s pretty clear it s one foot.
You add an extra 3 feet to the length.
15cm that s how far off the ground we re lifting the string remember out of 6 370km is close to one part in forty million.
This puzzle is somewhat counter intuitive.
All around the earth the rope is raised up uniformly as high as is possible to make it tight again.
If you put 1 metre high sticks right around the equator and lay the rope on top how much longer does the rope need to be to make ends meet.
Divide again by pi to get the earth s radius 6 370km.