The Hoberman sphere is a structure resembling a geodesic dome, and which can fold down to a fraction of its normal size. The scissor-like action of its joints enables it to do this. It's impressive to watch. Some museums have this sphere on display, using a motorized control system to continuously change the size of the sphere from fully closed to fully open.
Check out a video of a Hoberman sphere:
If you get your hands on one, have a look how it works. Look at how the joints bend relative to each other when the sphere opens and closes. If you look at it closely you can see a pattern in how its structured. It's a beautiful toy both aesthetically and how it's put together. The geometry of the pieces have to work smoothly together and not interfere with each other, otherwise the sphere will bind up somewhere and it won't be able to open and close properly.
Does the sphere feel heavier when it's open or closed? Obviously, the weight is the same, but sometimes the shape of an object can affect the perception of weight.
You can play many different games and create many interesting effects with the Hoberman sphere. This is explained with the manual that comes with the sphere.
One really neat trick you can do with the sphere is shown in the video. The guy in the video spins the sphere while in its open position, while holding it between his hands. He then pushes the sphere into its closed position, and the sphere spins faster as a result. This is the same effect that's created when a figure skater pulls his arms in during a spin. It causes him to spin faster. In scientific terms, this effect is caused by conservation of momentum, which is a subject covered in physics classes.
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