Storm in a Teacup: The Physics of Everyday Life by Helen Czerski

Author:Helen Czerski [Czerski, Helen]
Language: eng
Format: epub
Tags: Science, Physics, General
ISBN: 9780393248968
Google: iUt8DAAAQBAJ
Amazon: 0393248968
Publisher: W. W. Norton
Published: 2017-01-09T16:00:00+00:00

7

Spoons, spirals and Sputnik

The rules of spin

ONE OF THE nice things about bubbles is that you know where to look for them: at the top. They’re either on their way there, wobbling upward through fish tanks or swimming pools, or nestled in with the crowd on top of champagne or beer. Bubbles reliably find their way to the highest point of the liquid they’re in. But next time you stir a mug of tea or coffee, have a look at what’s going on at the surface. The first odd thing that happens is that as you move the spoon around in circles, the surface of the tea develops a hole. As the liquid whirls around, the middle of the tea sinks and the edges rise up. And the second odd thing is that the bubbles in the tea are spinning quietly at the bottom of the hole. They’re not at the highest point of the tea, at the edges. They’re hiding at the lowest point on the surface, and they stay there. If you push them away, they find their way back. If you make new bubbles at the edges, they spiral into the centre. Odd.

When I start to stir my tea, I’m pushing on the liquid with the spoon. I push it forwards, 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 towards the centre 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

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