Ellipsoid paradox: This paradox considers a perfectly reflecting cavity with two black bodies at points A and B. The reflecting surface is composed of two elliptical sections E1 and E2 and a spherical section S, and the bodies at A and B are located at the joint foci of the two ellipses and B is at the center of S. This configuration is such that apparently black body at B heat up relative to A: the radiation originating from the blackbody at A will land on and be absorbed by the blackbody at B.

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"I got this for a gift and they were blown away by it! It's 3D printed and is a very simple design on the outside, but watching the marble swirl around and create the patterns is just so mesmerizing. It's fun to mess the pattern up and watch the ball just go back to work.

Here is a selection of four-star and five-star reviews from customers who were delighted with the products they found in this category.

"Just perfect! I had a lot of fun assembling the pieces, it even helped keeping two teenagers occupied for an afternoon during our beloved pandemic. Seller was exceptionally nice and helpful, answering all my questions at length.

"I have get more info been collecting unusual things for a long time and this perpetual motion machine is one of my instant favourites. Great quality, the hidden on/off switch adds to the illusion.

These 4 or 5-star reviews represent the opinions of the individuals who posted them and do not reflect the views of Etsy. The ratings/reviews displayed here may not be representative of every listing on this page, or of every review for these listings.

Here is another cool gift idea for those of you who have a thing or two for marble machines. William Le’s Perpetual Marble Machine has a wood and stainless steel construction.

Maxwell's demon: This was originally proposed to show that the second law of thermodynamics applied in the statistical sense only, by postulating a "demon" that could select energetic molecules and extract their energy.

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/20. This would then allow students to calculate the speed of the ball at different locations along its trajectory and quantitatively confirm the statement above that the ball obtains a burst of speed close to the bottom part of its trajectory.