As it turns out, a Hoberman sphere is also an excellent tool for demonstrating the conservation of angular momentum. In the video above, Utah State University’s Professor Boyd F. Edwards spins the expanded sphere, then pulls a string to contract it, causing “the sphere’s moment of inertia to decrease and its angular speed to increase.” From Khan Academy:
Angular momentum depends on the rotational velocity of an object, but also its rotational inertia. When an object changes its shape (rotational inertia), its angular velocity will also change if there is no external torque.
Once a skater leaps into the air, they’re stuck with however much angular momentum they’ve created. But a skater can change one thing: the moment of inertia. Moment of inertia determines how easy it is for an object to speed up or slow down, and describes the resistance that a force is working against. A larger moment of inertia—like when a skater extends their arms—will result in a slower rotational speed. But a smaller moment of inertia—like when a skater hugs their arms into their body tightly—will lead to a faster spin.
Watch more physics demonstrations on this site, including Demonstrations of the Coanda Effect, observing the angle of repose with DIY physics devices, Newton’s Beads, and how to make a self-starting siphon.