Before you are two balls, one solid and one hollow. They are to all appearences completely identical: same size, same weight, same outer material (though one might assume, correctly, that the hollow ball would need a higher-density material on the inside to make it the same weight).

Without breaking either of the balls, how can you easily determine which is hollow?

Assume that the material is solid enough that a hitting the side of the hollow ball will not result in any noticeable echo or vibrations.

• Take one cat and bind the first ball on the back of the cat.
• Get hold of the cats paws and lift cat and ball to +/- 2...3 meter (depends on the ball/cat mass ratio)
• Let go (Paws must be directed upwards).
• During the fall, the cat will use its tail to turn in the air and get with four paws on, the ground.
• If this happens, then repeat the test with the other ball.
• If this also goes fine, then repeat both tests from a lower altitude, untill the cat only succeeds with one of the balls.
• The other is the hollow one.
  

Theory note: The hollow one has the largest moment of inertia and this will make it the cat the most difficult to turn around.
PS: use only theoretical cats.

Edited on March 25, 2005, 4:55 pm

Posted by Hugo on 2005-03-25 16:54:36