### Brain Teasers

# Ring Around the Earth

Science
Science brain teasers require understanding of the physical or biological world and the laws that govern it.

Imagine a smooth metal ring running around the earth's equator about the width of a sidewalk. Now imagine a metal cable wrapped around this first ring so tightly that nothing can squeeze between the cable and the ring. You may assume that the ring and cable are perfect circles even though they most likely would never be. Now this cable, which does not stretch, has three extra feet added to it and is made to magnetically float up and off of the ring so that extra three feet of slack is then equally distributed around the earth. This now gives you an equal distance between the ring and the cable anywhere you checked it around the planet. Now before you do the math, if you are going to do the math at all, what do you think the largest object is that will fit between the cable and the ring?

A) an electron (Less than 1x10-13 cm)

B) a virus (~100nm)

C) single skin cell (~20 microns)

D) a grain of sand (~200 microns)

E) a typical pearl (~9mm diameter)

F) a grape (~1 inch diameter)

G) a grapefruit (~5 inches diameter)

H) a pumpkin (~1.5 feet diameter)

A) an electron (Less than 1x10-13 cm)

B) a virus (~100nm)

C) single skin cell (~20 microns)

D) a grain of sand (~200 microns)

E) a typical pearl (~9mm diameter)

F) a grape (~1 inch diameter)

G) a grapefruit (~5 inches diameter)

H) a pumpkin (~1.5 feet diameter)

### Hint

The earth's diameter is 41,865,458 feet or 12,760 km. Do the math first as if the cable were wrapped around a soccer ball say with a 1 foot diameter and then had 3 feet added to it. Then do the earth.### Answer

The gap comes to about 5.73 inches, which is about the size of a grapefruit! If you wrap the cable around a soccer ball and add 3 feet the gap between the soccer ball and the cable will be 5.73 inches, the same as it was for the earth. If you wrap the cable around the solar system and add 3 feet you also get a 5.73 inch gap! It does not matter what the original circumference is in this problem, the answer will always be the same!Hide Hint Show Hint Hide Answer Show Answer

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