Storm in a Teacup: The Physics of Everyday Life(65)

The first challenge was to throw as many boots as possible over a line about 80 feet away in two minutes. The top five teams would then carry on to the next stage of the competition, which was to see who could throw the farthest overall. The clock started. The three of us heaved on the rope, turning the seesaw and flinging the sling. But the first boot barely cleared our heads. We couldn’t pull downward fast enough to make the seesaw rotate properly. We tried again. And again. After about a minute, I convinced my teammates that this wasn’t working, and we set ourselves up for the original idea. I put my diving weights on, jumped off the small filing cabinet we were using as a platform, swung underneath the pivot, and wheeeeee . . . the first boot went soaring over my head and over the line. Next one. Boot in the sling, up on filing cabinet, swing down, whoosh! Next— but the whistle had gone. Our time was up. Two boots over the line wasn’t enough. We wouldn’t get through to the next stage. The middle-aged men commiserated with us. Better luck next time. I hid from the official who had suggested the rope, because I was so cross with him. It worked! Our simple structure of scaffolding and carpet and elegant physics worked, and it had worked the way I’d said it would. We could have competed against the lovingly painted, intricately made garage beauties! But we had been scuppered by the last-minute change of plan.§ Most of the other competition entries were based on far less efficient methods. They might have been colorful, but we had physical efficiency and simplicity on our side.

So my personal success with trebuchets is a bit limited, but eight hundred years ago this elegant idea revolutionized warfare. Being able to throw heavy rocks with high accuracy meant that you could thump the same bit of the castle wall again and again, until it yielded. For two centuries or so trebuchets got bigger and better, and were given names like “God’s stone thrower” and “Warwolf.” Each one took vast amounts of timber to construct, but being able to chuck another 330-pound rock at your enemies every few minutes was worth it. Spinning the rock and the sling around an axis lets you build up to a very high speed in a very short space of time. You don’t want the spinning to continue—you’re just using it as a way of reaching a high speed. Once the projectile is going fast enough, you remove the inward force at the moment when its direction is perfect. And off it goes, soaring out in exactly the direction it was released in. Until gunpowder became reliable enough for the cannon to be more of a weapon than a liability, in terms of destructive efficiency a trebuchet was as good as it got.


LOTS OF THINGS are spinning. For example, right now, you and I are spinning. We’re going around the axis of the Earth once a day, although we can’t feel it because the Earth is so big that we’re only changing direction slowly. If we were at the Equator, our sideways speed would be 1,040 mph. In London, where I’m writing this, I’m speeding sideways at 650 mph because we’re closer to the spin axis. But if we all live on a massive spinning planet, and if a loose object at the surface of a spinning thing will whizz off in a straight line when you let go of it, why are we all still down here? The answer is that the inward pull of gravity is strong enough to prevent the planet letting go of us. In fact, even when you’re in orbit, the planet hasn’t actually let go of you. And when you’re on your way up there, that extra speed you’ve got because of the Earth’s spin can be extremely useful.

On October 4, 1957, a diminutive metal sphere named Sputnik chirped out the first sounds of the Space Age, and the world listened open-mouthed. Earth’s first artificial satellite was a huge technological achievement. Sputnik orbited its home planet once every ninety-six minutes, and each time it went past anyone with a short-wave radio they could hear its distinctive “peep . . . peep . . . peep. . . .” America had woken up that morning happily complacent in the knowledge that it was the greatest nation on Earth. It went to bed shocked that maybe it wasn’t. Within a year, the Soviets had sent up Sputnik II, a bigger satellite carrying a dog named Laika. The panicked Americans hadn’t sent anything into space, but they had launched NASA, the National Aeronautics and Space Administration. The Space Race had begun in earnest.

But what was the real achievement of Sputnik? It wasn’t just about going up; anything close to something as big as a planet has to live up to the maxim “what goes up must come down.” The trick of putting satellites into orbit starts with what goes up, but the real skill is in delaying their coming down for as long as possible. Sputnik hadn’t escaped the Earth’s gravity. That wasn’t the point. Douglas Adams summed it up perfectly, and accurately, with the minor caveat that he was talking about flying and not orbital space flight: “The knack lies in learning to throw yourself at the ground and miss.” Sputnik was permanently falling toward the Earth. It just kept missing it.

Sputnik was launched from the deserts of Kazakhstan, a place that is now the site of the Baikonur Cosmodrome, a vast space launch facility. The rocket that carried Sputnik powered upward through the thickest part of the atmosphere and then turned sideways, accelerating horizontally around the curve of the Earth. By the time the last parts of the rocket fell away, Sputnik was whizzing around the planet at about 5 miles every second, or 18,000 mph. This is where the effort goes when you’re getting into orbit—it’s mostly about going not upward, but sideways.

The little metal sphere hadn’t escaped gravity at all. In fact, it needed gravity to be there, to make sure that it stayed in orbit and didn’t just carry on and leave the Earth behind. As it zoomed along at this fantastic speed, the Earth was pulling it downward with almost as much gravity as there is on the ground.? But because Sputnik had such a huge sideways speed, by the time it had fallen a little way down toward the Earth, it had gone so far forward that the Earth had curved away beneath it. And as it kept falling, so the Earth’s surface kept curving away. This is the beautiful balance of being in orbit. You’re going sideways so quickly that you fall toward the ground and miss. And because there’s almost no air resistance, you can just keep falling and missing, as you go around and around.