What happened next happened very quickly. As the circle of dough left his hand, it was suddenly free of anything external pulling and pushing on it. It’s helpful to think about a single point on the edge. It’s traveling around in a circle, but only because the rest of the dough is stuck to it, pulling it inward. That inward pull is always necessary for something to rotate. In the case of the cyclist, the track is constantly pushing the bike from the outside, so that the cyclist must curve inward toward the center instead of continuing on a straight line. For the pizza dough, it’s a pull from the middle that makes the edge of the dough curve around toward the center. Either way, there has to be a force directed toward the middle of the spin. But dough is soft and elastic. If you pull on it, it stretches. The middle of the dough is pulling the edge inward, but that means there’s a pulling force across the dough. And so the dough has to stretch. When any solid object spins, the spinning generates forces inside it that you can’t see. The internal pull that’s keeping the pizza together is also stretching the dough, and the edge is getting farther and farther from the center. The brilliance of this for a pizza chef is that the internal pull is smooth and symmetrical. All of the pizza is spinning, so all of it stretches out away from the center.
You can feel these internal pulling forces yourself sometimes. If you hold a bag containing a reasonably heavy object out horizontally, and spin yourself around, you’ll feel a pull trying to stretch your arm. This is the inward pull that’s keeping the bag spinning in a circle. Fortunately for you, your arm is much less stretchy than pizza dough, so it stays the same length. But the longer your arm and the faster you spin, the more of a pull you’ll feel.
So as the pizza dough was spinning in the air, the same pull that was keeping the edge moving around in a circle was gradually stretching the dough outward. I reckon the dough was in the air for less than a second, but it was a very thick pancake when it went up, and a beautiful thin smooth circle when it came down. The chef kept it spinning and sent it up again, but this time the internal pulling forces were so strong that the dough tore itself apart in the middle, and what came down was sad and raggedy. The chef grinned sheepishly. “That’s why we don’t normally do it,” he said. “The dough that makes the best pizza is too soft to spin, so we have to stretch it by hand on the board.” ? It turns out that the dough used in the acrobatic competitions is made using a special recipe so that it’s stretchy and strong, but doesn’t necessarily produce the cooked pizza with the best texture. Right at the edge of a pizza, the internal pulling force could be five to ten times as strong as gravity, which is why the pizza base stretches much more quickly when you spin it than it does if you just hold it up and let it droop under its own weight.
A spinning pizza base is lovely to watch because it changes shape in response to forces that are entirely hidden away inside itself. Spinning anything generates a pulling force from the center to the edge—the same is true in a spinning rugby ball or Frisbee—but you’d never know about it in these solid objects because they’re strong enough to resist being stretched. Or at least, they’re stretching so little that you can’t tell. But everything will stretch a tiny bit. Even the Earth itself.
OUR PLANET IS constantly spinning as it travels around the sun. And, just like the pizza dough, it’s stretched by the forces that are pulling each bit of it back inward, keeping each bit of rock traveling in a circle. Fortunately for all of us, gravity is strong enough to prevent any consequences as extreme as they are for the dough, and the Earth stays fairly spherical. But it does have what is helpfully called an “equatorial bulge,” which sounds like a euphemism for having eaten too much cake. If you stand at the Equator, you’re 13 miles farther away from the center of the planet than someone at the North Pole is. Our planet is held together by gravity but shaped by its spin. And so even though Mount Everest is the tallest mountain on Earth, the top of Everest is not the farthest point from the center of the Earth. That accolade goes to Chimborazo, a volcano in Ecuador. Its summit is only 20,564 feet above sea level (Everest is 29,029 feet tall by the same measure), but it’s sitting right on top of the equatorial bulge. So, when you’re standing on top of Chimborazo, you’re a little over 1.24 miles farther from the center of the Earth than anyone who has just struggled to the top of Everest. Pointing that out when you both get home probably won’t make you popular, though.
Overall, the forces generated by spinning can be useful in two ways. The pizza is one—spinning something without confining it generates a pull inside the object as it tries to hold itself together while it spins. The cyclist is the other—if you do put a wall in the way, confining whatever is spinning with something that pushes back, you can generate a strong consistent gravity-like force on that object. But the common theme is that an inward pull or push has to come from somewhere. If that inward force ever goes away, the object can’t stay on its circular path.
Only a solid object can hold itself together like the pizza dough. Liquids and gases aren’t stuck together like that.? This distinction is fantastically useful if you have both solid objects and liquids mixed together, because now you can separate them out. The brilliance of a spin dryer for clothes is that the clothes are trapped inside the drum, and the drum is pushing them inward so they have to keep going around in circles. But the water tucked away in the clothes isn’t held in position. Since it’s free to move, it can keep moving outward through gaps in the material. It will only travel in a circle if it gets an inward push from something solid. Otherwise, it will gradually wriggle its way away from the center, and when it meets a hole in the drum, it’ll go flying out sideways, free of the circle completely.
When you spin something around and then let go, you start by pulling on it with exactly the right inward force to keep it going around in a circle, and then you suddenly take that force away. When there’s no inward pull, there’s no reason for the object to keep going around in a circle. So it just sails off in a straight line. This principle revolutionized medieval warfare in Europe and the eastern Mediterranean, enabling engineers to build giant siege engines that could batter down stone fortresses. And I have used it to launch boots, but not quite as effectively.