A hoberman sphere can be used to demonstrate conservation of angular momentum. The sphere has a pulley attached to it and has a string runs through its center. The string is attached to the bottom link, goes up through the pulley and back down through the bottom link out of the sphere. This allows you to pull the string and make the sphere contract while it is hanging from a string. If we give the sphere some rotational velocity while it's hanging and pull the string so that it contracts, we'll see the sphere speed up due to conservation of angular momentum.

This sphere can also be used to demonstrate centrifugal forces in rotating body. Placing it on a rotating platform while it is contracted and giving it some rotational motion, one can see the the sphere expand, suggesting that some radial, centrifugal force is acting on the sphere, pulling the links outward.

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