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Hollow ball (Posted on 2005-03-25) Difficulty: 3 of 5
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.

See The Solution Submitted by Sam    
Rating: 3.6667 (3 votes)

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Summary | Comment 15 of 26 |

Several methods have been proposed, and I propose a few new ones.

I think the author was going for methods based on the difference in moment of inertia, so spinning or rolling down an incline should be successful in distinguishing the 2 spheres.

Floating won't work since the 2 spheres have identical mean densities (the air or vacuum inside the hollow sphere is offset by the fact that the metal of the hollow sphere is more dense than the metal in the solid ball)

Ultrasound sonography should work (unclear if this violates the "hitting the side of the hollow ball " rule or not )

X-ray or CAT scan should also work if the energy of those high energy photons is high enough.

Bouncing the spheres off of a solid, very hard surface might show a difference, but I don't think we have enough info on the mechanical properties of the materials to know this for sure ( I would expect the hollow ball to bounce higher)

  Posted by Larry on 2005-03-27 02:11:27
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