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Magnet Race

Science brain teasers require understanding of the physical or biological world and the laws that govern it.


Puzzle ID:#25825
Fun:*** (2.78)
Difficulty:*** (2.36)
Submitted By:phrebh*us****
Corrected By:drussel3




Fred and Bob are racing cylindrical magnets down tubes of various materials. All of the magnets weigh the same and have the same dimensions. All of the tubes are the same length and have an inner diameter just larger than the outer diameter of the magnets. There is negligible friction when the magnets travel through the tubes.

Since they don't want the magnets to get stuck, Fred and Bob are using non-ferrous materials that magnets won't stick to. They have one tube for each material: wood, glass, aluminum, and plastic. Each of the tubes is set up at the same angle with respect to the floor and they are all in the same room on Earth.

Assuming they have a perfect method of timing each run, which tube has the longest time?


The aluminum tube will give you the longest time. Even though aluminum is non-ferrous, it is still a good conductor, and therefore feels the induced flux of the Faraday effect.

The moving magnet causes a current in the aluminum tube, which in turn creates a system wherein the magnet is attracted to the non-ferrous aluminum, causing it to slow down.

If you are in secondary school or college, the physics department will probably have a demonstration that shows exactly this. The demos are sold with two cylinders that look alike and weigh the same, but one is a magnet and the other isn't. The idea is that you can see how the magnet falls through the tube at a slower rate than the non-magnet.


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Sep 08, 2005

I'll assume you are correct.
Sep 08, 2005

Did you study for that? I never could have done it!
Sep 08, 2005

I have a physics degree, so it was easy to come up with!
Sep 08, 2005

Zoinks! Who knew?? I guessed wood, thinking it would have more friction. I learned something!!
Sep 09, 2005

I actually knew that, I sacre myself.
Good job with #1 place for Click the Button
Sep 11, 2005

my head hurts
Sep 12, 2005

anyone ever tell you you know too much phrebh
good teaser, i guessed alumium because it was a metal, but had no real idea :/
Sep 13, 2005

i love science and this is proof... im thirteen and i got this right no offense to anyonwe or nething but good teaser
Sep 30, 2005

I knew it.
Wood? Why would magnets go slower in wood? Wood has absolutley nothing to do with magnets, or at least not that I know of. Aluminum lasting longer makes more sense; aliminum is a type (actually, correct me if I'm wrong, but isn't aluminum a man-made thing??) of metal.
Oct 04, 2005

Physics class flashbacks!! *will need therapy*
Great job!
Oct 07, 2005

I GOT IT!!!!!!!!!
Oct 07, 2005

Oct 15, 2005

Oct 26, 2005

Nov 02, 2005

very, very GOOD
Nov 09, 2005

WELL. i also thought wood, so dont feel bad, Aarghonauts! i actually figured aluminum would be one of the faster ones cuz it was smooth. ha u guys had better hope i dont build rockets or somehting one day. i'll probably blow up half the planet.
Jan 12, 2006

umm... smoothness doesn't matter he said friction was negligable. This was probably one of the best ones I've seen on here. Good teaser.
Jul 19, 2006

The magnet is not attracted to the aluminum tube - rather, the magnet passing down the tube creates an electric current going clockwise as viewed from above, which in turn creates a magnetic field opposite to the falling magnet (has to do with flux, too difficult to explain here). If it's a thick tube and a strong magnet, it will fall VERY slowly.

A similar principle is used in Europe for magnetic brakes on trains. They simply have an electromagnet next to the wheels, and they simply turn them on to stop the train. The magnets set up "eddy currents", which, in addition to the magnetic field, slow the wheels down. The advantage to this system is that the stopping force is proportional to the velocity of the wheels, which means that the train will come to a smooth stop without lurching at the end.
May 03, 2007

Actually, flux is a measurement of magnetism, which in turn is defined as a phenomena of attractiveness (positive or negative) of multiple materials. Therefore the magnet *is* attracted the aluminum tube.
Jun 15, 2007

I'm not quiet at te physics level, but I just assumed it would be aluminum because of an electromgnetic field, or maybe because it was similar to metal. Even though I'm ahead of my class I'm still simplifying algebraic equation and makin sure the those suckers the numbers and variables stay on their side.
Jun 15, 2007

I'm not quiet at te physics level, but I just assumed it would be aluminum because of an electromagnetic field, or maybe because it was similar to metal. Even though I'm ahead of my class I'm still simplifying algebraic equations and making sure that those suckers the numbers and variables stay on their side.

Sorry for the double post. Pet peeve on leaving bad spelling. Argh.

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