Storm in a Teacup: The Physics of Everyday Life(17)
The road between the two towers splits into two separate halves. About a thousand times each year, a ship or a boat arrives at the bridge, and the traffic is stopped. Each half of the road swings upward, and on the other side of the pivot, in this dark chamber underneath the tower, the hidden half of the bridge swings downward. I look upward at the underside of this seesaw, and ask what exactly is hanging above us. Glen, our guide, is quite cheery about it. “Oh, there’s about 460 tons of lead ingots and bits of pig iron up there,” he says. “It rattles around loose—you can hear it when the bridge opens. When they change anything on the bridge, they usually add a bit or take some away so it stays perfectly balanced.” We are apparently standing directly underneath the biggest beanbag in the world.
It’s the balance that is the key. Nothing ever lifts the bridge. All those engines do is tilt it a bit—what’s on one side of the pivot point is exactly balanced by what’s on the other side. This means that very little energy is required to move it—only just enough to overcome the friction of the bearings. Gravity effectively goes away as a problem, because the pull downward on one side is exactly balanced by the pull downward on the other side. We can’t beat gravity, but we can use it against itself. And you can make a seesaw as large as you like, as the Victorians recognized.
After the tour, I walked a little distance along the river and then turned around to look back at the bridge. My view of it had completely changed, and I absolutely loved seeing it differently. The Victorians didn’t have electricity on tap, computers to control things, or swanky new materials like plastic and reinforced concrete. But they were masters of simple physical principles, and the simplicity of the bridge really gets to me. It’s precisely because it’s based on something so simple that it’s still working after 120 years, with almost no alteration. The gothic revival architecture (which is apparently the technical term for “fairy-castle style”) is just wallpaper covering up a giant seesaw. If they ever build one like this again, I hope that they make some of it transparent, so that everyone can see the genius of it.
This trick for reducing the problems of gravity can be seen all over the place. For example, imagine a pivot point 4 yards above ground with two 6-yard-long halves of a seesaw balancing each other out on either side. This isn’t a bridge. This is a Tyrannosaurus rex, the iconic carnivore of the Cretaceous world. Two chunky legs hold it up, and the pivot point is at its hips. The reason it didn’t spend its life falling flat on its face is that the large heavy head with its terrifying teeth was balanced out by a long, muscular tail. But life as a walking seesaw comes with a problem. Even a very determined T. rex would sometimes have needed to change direction, and they were lousy at it. It’s been estimated that it could take one or two seconds for them to turn through 45 degrees, making them a bit more cumbersome than the clever, agile T. rex of Jurassic Park. What could limit a huge, strong dinosaur in such a way? Physics to the rescue. . . .
Spinning ice-skaters bring many things to the world—aesthetics, grace, and astonishment at what the human body is capable of. But if you hang around physicists explaining things often enough, you could be forgiven for thinking that their sole contribution is to show everyone that sticking your arms out makes you spin more slowly than when they’re tucked in. They’re a useful example because ice is more or less free of friction, and so once somebody is spinning, they have a fixed “amount” of spin. There’s nothing to slow them down. So it’s really interesting that when they change their shape, they also change their speed. It turns out that when things are farther from the axis of spin, they have to travel farther on each turn, and so effectively take up more of the available “spin.” ? If you stick your arms out, they’re farther away from the axis, and so the speed of rotation goes down to compensate. And this is basically the problem that the T. rex had. It could only generate so much turning force (“torque”) with its legs, and because its huge head and tail were sticking out just like very fat, heavy, scaly versions of the skater’s arms, it could only turn slowly. Any small agile mammal (for example, one of our very distant ancestors) would be a lot safer once it had worked that out.
The same thought also explains why we put our arms out sideways when we think we’re about to fall over. If I’m standing upright and I start to fall to my right, I’m rotating around my ankles. If I stick my arms out or up before I start to fall, the same tipping force won’t move me as far, and so I’ve got more time to make adjustments to stay standing up. That’s why gymnasts on the balance beam almost always have their arms out horizontally—it’s increasing their moment of inertia, so they’ve got more time to correct their posture before they fall too far. Having your arms out also lets you rotate yourself by lifting or dropping them, and that helps your balance, too.
In 1876, Maria Spelterina became the only woman ever to cross Niagara Falls on a tightrope. There’s a photograph of her halfway across, serenely balanced and with peach baskets attached to her feet (to increase the drama). But the most obvious prop in the photo is the long horizontal pole she’s carrying, the best aid to balance. Arms will only reach out so far, but this arm-substitute was a large part of the reason for Maria’s exquisite control.# Her T-shape meant that if she started to lose her balance, it would only happen very slowly, because the ends of the pole mean that the same torque has less effect. Maria was concerned with falling over to one side, but the long pole would also have made it very difficult for her to twist from left to right. And so it was with the T. rex. The same bit of physics that was Maria’s best defense against falling 160 feet to certain death in the churning water had also, 70 million years earlier, made it impossible for a T. rex to change direction quickly.