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        riddles >> easy >> Three metallic balls
        
(Message started by: phobos on Nov 28^th, 2003, 7:40am)

Title: Three metallic balls
Post by phobos on Nov 28^th, 2003, 7:40am

Here's a small puzzle that a friend of mine was asked in an interview for his application to the engineering graduate studies in University of Southampton.

There're three metallic balls. One is made of aluminium, one is made of iron (hollow core), and one is made of aluminium-iron alloy. All of them are in the same size, weight the same and look the same. How would you differentiate them non-destructively?

Title: Re: Three metallic balls
Post by LZJ on Nov 28^th, 2003, 8:32am

[hide]Use a magnet to determine the aluminium ball. For the hollow ball and alloyed ball, roll both down a slope simultaneously. The one that rolls down faster should be the solid one, (higher angular acceleration due to lower moments of inertia), the slower is the hollow ball. [/hide]

Title: Re: Three metallic balls
Post by Lightboxes on Nov 28^th, 2003, 8:48am

::[hide]
gj on the rolling part.
But I say use a magnet on the iron ball. The aluminum-iron alloy is non magnetic even with iron in it, as long as the iron is not very high in percentage amounts...say 50/50.

Aluminium is a lot lighter (in weight) than iron. So the aluminum-iron has to have a hole in it too as well to have the same size and weight as the others...as the iron one does too.
So you could roll all of them and the iron would roll the slowest, the aluminum-iron second slowest, and the aluminum the fastest, as LZJ has already mentioned. Maybe even spin them on their axis and see how hard it is to spin or how long they spin.
[/hide]::

Title: Re: Three metallic balls
Post by towr on Nov 28^th, 2003, 11:49am

I think I'd just try to distinguish them on touch.. aluminium and iron feel quite differently.. Also, tasting them could work.. (I'd definately taste which one had any iron.. Note: when I say tasting, I don't mean taking a bite out of it, just licking it (after maybe cleaning it it a bit, never know where it's been))
Another way would be to use electrical conductivity..

Title: Re: Three metallic balls
Post by Icarus on Nov 28^th, 2003, 12:02pm

Kozo already posted one like this (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_easy;action=display;num=1049700553). Several possibilities were mentioned, but the one most generally agreed on as unfailing was the spin rate, since the moments of inertia of the three spheres will differ.