Flywheel

Try the law of conservation of angular momentum for yourself! Stand on the rotating platform and spin the flywheel around a horizontal axis. Then tilt it using the handle. What happens?

Tilting sets the platform itself turning. Why? When the wheel is held perpendicular to the platform, its angular momentum has only a horizontal component. The moment you tilt the wheel, that horizontal component — or part of it — becomes vertical. But for angular momentum to be conserved, another vertical component must appear, pointing the opposite way, so that the two still cancel out. And that shows up precisely as the platform turning the other way.

It may look like a party trick, but flywheels — gyroscopes — have practical uses in navigation and positioning devices, in clockwork, and as energy stores. Energy accumulated in a flywheel can even power buses. And if you have ever ridden a Segway, know that a flywheel helps keep it stable, evening out sudden changes.

The same law explains why a figure skater spins faster when drawing their arms in and slows down when spreading them out — try it yourself on the ice! It is also why stellar remnants such as neutron stars rotate so fast: they are minuscule compared with the original star yet still carry most of its mass.

As you can see, physics is all around us — and at the Institute of Physics we get to the bottom of it. We study cosmic rays, some of which reach us from neutron stars, and we help build space probes for which gyroscopes are a crucial component. We are building part of the LISA space mission, which will search for gravitational waves from neutron star collisions — and perhaps even from the era of the Big Bang. But that is a story for another time!