Tennis Balls & Planets
Math brain teasers require computations to solve.
A fool wants to tie a rope around the earth. So he buys a rope of 40,000 KM and ties it around the world. His neighbour, also a fool, wants to do the same only he wants the rope on sticks 1 meter above the ground.
How much more rope does he need?
And how much more rope do you need when you use a tennis ball instead of the earth?
Answer
6 meters 28 centimeters.
x=2*PI*(r+1)2*PI*r
x=2*PI*r+2*PI2*PI*r
x=2*PI
x=6.28
It does not matter what the radius of the circle is. You always need 2*pi meters more of rope for each additional meter of radius added.
Hide
Comments
rayneeday
Mar 08, 2002
 Come on, tell me what you think. I am of the opinion that the Comment box is way under used. If you like it great, If you hate it, tell me. So I can fix the problem in future puzzles. Personally I think more feedback both good and bad can only help the website produce more fun for all. Even a little trash talkin' can be fun.

captbob61
Mar 28, 2002
 It absolutely DOES matter what the radius of the earth is! The only time that "2*pi" applies is when you are increasing the applied radius by 1 unit. Still, keep them coming, you are my favorite riddle writer at Braingle! 
Lyceum_
Apr 03, 2002
 No, the radius of the original ball does not matter. Adding 6.28 meters of rope will always increase the diameter of the circle by 2 meters, which will make the rope 1 meter off the earth (or tennis ball) all the way around. 
Lyceum_
Apr 03, 2002
 Think of it another way: suppose there was no tennis ball, and there was just a tiny knot of rope. To make a circle 2 meters wide you need 6.28 meters of rope. The reason it doesn't seem right is that we are used to perceiving the AREA of a circle instead of its CIRCUMFERENCE. 
NuttyPrince
Jul 06, 2002
 huh? er...good teaser...Nutty 
Gizzer
Jul 19, 2002
 Actually, even a fool wouldn't need more than than 40,000 km of rope to tie around a mere tennis ball! 
Jimbo
Apr 21, 2009
 The answer is quite correct. It asks for how much MORE rope is needed and the same extra amount is required no matter what. If you want an explanation that uses calculus then given C = 2*Pi*R then dC = 2*Pi*dR which tells us that the change in circumference is 6.28 times the change in radius and there is no reference to the size of the original radius. It's an oldie but a goodie! 
Back to Top
 
