When I start to stir my tea, I’m pushing on the liquid with the spoon. I push it forward, but there’s only so far that it can go before it meets the side of the mug. If I did the same thing with a spoon in a swimming pool, the water in front of the spoon would move forward, and it would keep going forward until it mixed in with the rest of the pool. But in the mug, there’s no room for that to happen. Even though the side of the mug isn’t going anywhere, it can still push back on any liquid that bumps up against it. It’s a wall, and tea can’t pass through it. Since the tea can’t go in a straight line, it starts to move around the cup in a circle. But as that’s going on it’s piling up against the walls because only the side of the mug can push back. The tea will keep trying to go in a straight line, and it only moves around in a circle because it’s being forced to curve.
This is the first lesson about spinning things. If you suddenly freed them of their restriction, they would just keep moving in the same direction they were going in at the moment of release. Imagine a discus thrower, spinning around while holding on to the discus. After a few rotations, the discus is going incredibly quickly, but it stays on its circle because it’s being tightly held. The athlete must continually pull it back toward the center of the spin, and that pull is along the line of his or her arm. The second that they let go, the discus travels forward in a straight line, with exactly the direction and speed it had before the release.
As I’m stirring my tea, the hole develops because each bit of tea tries to move in a straight line, but that makes it push up against the outsides and so there’s less left in the middle. When I stop stirring, the hole remains because the liquid is still spinning. As the whirling slows down, it takes less of a push to keep the tea going in a circle, and so there’s less of a pile-up at the sides. You can see all this in a liquid because it’s free to move, so it can alter its shape.
And at the center of the circles, the bubbles are spinning away. What their presence in the middle tells you is that it’s the least favorable place to be. When a glass of beer is sitting on a table, the bubbles move to the top because the beer is winning the competition to get closest to the bottom. And it’s the same for the mug of tea. The bubbles are in the middle because the tea is winning the competition to move out to the sides. The liquid is more dense than the gas, so the gas drifts into the space left over.
Our civilization is full of things that spin—clothes dryers, discus throwers, flipped pancakes, and gyroscopes. The Earth itself spins as it circles the sun. Spinning is important because it lets you do lots of interesting things, sometimes involving enormous forces and oodles of energy, all without actually going anywhere. The worst that can happen is that you end up back where you started. The bubbles in the tea are just the start. The same principle also explains why you wouldn’t launch a rocket from Antarctica and how doctors measure whether you have enough red blood cells. Spinning could also play an important role in our energy grid in the future. All of those possibilities come from a restriction: the one thing you can’t do when you’re spinning is to travel in a straight line.
If you’re zooming around in a circle, there must be something either pulling or pushing you inward, forcing you to change direction constantly. That applies to anything that’s spinning, whatever the situation. If that extra force is taken away, you just continue in a straight line. So if you want to travel in a circle, you must have something to provide an extra, inward push. The faster you’re going, the stronger that push has to be, because the faster you need to curve, the more force it takes. Spectator sports love a good racetrack; they have the same benefit as anything else that spins. You can achieve huge speeds without really going anywhere, and certainly not anywhere the paying audience can’t see you. To make sure that they get enough of an inward push to stay on the track, some sports have taken racetrack building to fairly extreme lengths. Indoor track cycling is the prime example of this. But it wasn’t the lengths that terrified me when I tried it . . . it was the steepness.
I’ve been an enthusiastic cyclist all my life, but this was a very different kettle of fish. The inside of London’s Olympic velodrome is shiny and vast and oddly quiet. You pop up into the middle of the stillness, and they issue you with a lean, mean-looking bike with a single gear, no brakes, and the most uncomfortable saddle I’ve ever had to sit on. When the group for the beginners’ session was assembled, we clopped outward to the track and held on to the rail while we clipped into the pedals. The track seemed enormous. There are two longer straight sides, and then the sweeping banked sections at each end that towered over us. They are so steep (43° in places) that it seems as though the designer had really wanted to build a wall. Cycling looked like entirely the wrong thing to be doing here. But it was too late for our little group now. The track was waiting for us.
First of all, we were sent around the flat oval that sits inside the main track. The surface was beautifully smooth, and the bikes made a lot more sense. Then we were instructed to venture outward onto the light blue strip with the first slight gradient. And then, feeling slightly like baby birds being pushed out of their nest to learn to fly, we were told to face the main track.
There was a nasty surprise straightaway. I had thought the banking would be gradual, but it isn’t. The gradient at the bottom is pretty similar to the gradient at the top. As soon as you stray outward onto the race surface, you’re cycling across a pretty significant slope. Pedaling faster seemed like a good idea, but that was only because I was forcing my brain to let logic make the decisions while it busily pretended that instinct didn’t exist. I forgot how stupidly uncomfortable the saddle was after the first three laps. Round and round we went, like demented hamsters on a gigantic wheel, pausing occasionally so that the instructors could check on us. Twenty-five minutes in, I was still terrified, but I was learning.