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.

Just spin the 2 balls, then just like 2 eggs, one hard boiled and one raw, the hollow ball will behave differently than the solid ball.  I assume the hollow ball will rotate at a slower speed for a given applied rotational force, but then will maintain its angular velocity better than the solid ball.

re: the floating method
The 2 spheres will both not float.  They have the same total mass and total volume and therefore the same average density.   Besides, the only things that float are: 
wood, apples, gravy, very small rocks, a duck, and witches.

(http://perplexus.info/show.php?pid=2860&cid=21169)

Posted by Larry on 2005-03-25 17:50:50