Ex Die in DIEM

by duncan mcnicholl

Angular Momentum

As part of the permanent collection at the Glasgow Science Centre, there is a circular platform which is mounted on ball bearings to allow it to turn easily. Attached to this platform is a bicycle wheel mounted by its hub, allowing it to be spun up to a nice high speed and then tipped from side to side. The whole setup is there to help people to understand the conservation of angular momentum: when the wheel is spinning and you try to tip it, the entire platform on which you are standing rotates in such a way angular momentum of the wheel, the platform and you is conserved.

You might have been given the chance to play with a similar setup at a science museum or science festival, or perhaps you’ve seen its close cousin, a freely revolving office chair and some weights, where the occupant of the chair holds the weights out to their sides while someone spins them, and can hugely increase the rate of spin by pulling the weights in towards themselves. It’s a lovely demonstration, but doesn’t have an immediately obvious use.

Esoteric uses abound, however: modern ships use something good called a gyrocompass which operates along similar lines, maintaining its orientation as the ship turns around it and so providing a reference for navigational purposes. The International Space Station and a variety of space telescopes use reaction wheels or control moment gyroscopes to efficiently change (or maintain) direction in space, allowing long exposure photography or uninterrupted communications to occur. The ISS is really using the gyros to ensure that the most robust part of the station faces in the direction of its orbit to increase its chances of survival in the chaos of low earth orbit, but the principal is the same.

It’s not just angular momentum that we store in wheels, though: we can use them to store energy too. Sometimes this is on a very short timescale, like a flywheel on a reciprocating engine absorbing the excess energy from a piston stroke and returning it to the crankshaft when the piston is idle, and sometimes it is over a longer period, such as when it is used to provide very high currents in fusion reactors, or as grid energy storage for load-balancing electricity grids.

On of the coolest uses of a flywheel for energy storage, however, was the KERS used in formula one starting in the 2009 season. It sucks up kinetic energy under braking, and adds it back to the driveshaft under acceleration, giving a power boost without using any extra fuel. The rules don’t say that KERS have to be flywheel based, and in fact all of the teams now use electrical systems instead which is sad from a mechanical engineering standpoint. The reason for the move away from flywheels is essentially that in a crash, nobody really wants a deliberately heavy chunk of metal spinning at sixty thousand rpm to come flying out of the car and tear its way through walls, the track and the crowd.

And now we’re getting to the point I wanted to make, I think. Sometimes it feels like there’s a part of our brain – or maybe heart – that acts like a flywheel. We spin ourselves up and up, even sometimes without realising what we’re doing, but adding more and more speed with stress and worry and anxiety, or fear and anger and confusion, and the flywheel just gets faster and faster and faster.

And with this flywheel spinning away, we end up trapped, in a way: all of that angular momentum means that we can’t turn or alter our path, even when we know that it’s the best thing to do, even if we think that the only way to dispel the momentum is to take a break, or break into a new heading, change our point of view in search of some clarity. And if that happens, you can end up on a collision course, unable to turn away as you barrel into events and interactions that would leave you battered and worn under the best of circumstances. Except these aren’t the best of circumstances.

If you come off the rails and crash with that great flywheel humming away inside of you, full of restless energy and everything you’ve been absorbing and not dealing with, all of that momentum that has nowhere else to go, and the whole thing might just drag its way out, tearing you up as it crashes through you, and spinning off to cause chaos while all you can do is watch.

And that’s why it’s so important to make sure that your flywheel isn’t carrying too much load, and to be aware of it, and keep it in check while you still can.