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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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