Interestingly, there is really only one thing you can do that might change the outcome,?? but it does bear a significant risk of unintended consequences. Just as you realize you’ve knocked the toast, just as it starts to teeter on the edge, the physics suggests that giving it another good sideways swipe will actually help. The toast will end up on the other side of the room, but because it spends less time pivoting on the edge, it won’t be spinning nearly as fast as it falls and may not rotate enough to turn over before it lands. So it stands a decent chance of landing buttered side up—but there is also a decent chance that it will end up under the sofa or stuck to the dog.
Toast starts to spin because it has two things going for it: a point to pivot around, and a force that’s pulling the toast around the pivot. It doesn’t matter that the force only points straight down and doesn’t keep pulling the toast around the circle. What matters is that the force is sufficient to move the toast (which it is, if the center of mass is over air and not table), and that it pulls the toast around the pivot for a little while at least. Once the spin has started, it’ll keep going until something stops it.
This is the same principle behind the spinning eggs in the introduction. If you think about the many things that spin freely—Frisbees, tossed coins, rugby balls, spinning tops—you’ll notice that they just keep going. It would be really odd if you flicked a coin so that it spun up into the air, and then somehow stopped spinning before you caught it.?? Anything that spins has angular momentum, which is a measure of its amount of spin. Unless something (such as friction or air resistance) slows it down, the object will spin indefinitely. This is the law of conservation of angular momentum. Something that is spinning will keep spinning, unless something happens to stop it.
I’m pretty sure that when I was a kid, dizziness was considered a sort of internal toy. If you were bored, you could always spin around on the spot, partly to see who could keep going the longest, and partly because it was funny that everyone fell over as soon as they stopped. The spinning itself didn’t seem to cause too many problems—the brief and entertaining disorientation comes when you stop. It’s a shame that adults don’t play this game very often; we might understand ourselves better if we did. The disoriented feeling happens because of something going on inside your ears that you can’t see but your brain is certainly aware of.
Let’s go back to spinning raw and boiled eggs, as I was talking about in the introduction. Each egg, still in its shell, is put down on its side and spun around. After a few seconds of both eggs spinning, you quickly put your finger on the shell of each to stop the rotation. Both eggs stop spinning. You take your fingers away. Then one egg starts revolving again. The egg that is solid has to stop spinning completely when you stop the shell. Both egg and shell have to move together. But when you stop the raw egg, you only stop the shell. The fluid inside is still spinning around; it’s not connected to the shell, and so there’s no reason for it to stop spinning. So the fluid pushes on the shell until the shell starts to rotate again.
When you spin yourself around, most of you (fortunately) is like the hard-boiled egg. It all has to move together. So when you stop spinning, your brain and nose and ears all stop, too. But not your inner ears. There are small, semicircular canals in each ear that are filled with fluid precisely because that makes them behave like the raw egg. The fluid doesn’t have to match the movement of its container because it’s not attached to it. This is one of the ways that your body senses where you are; tiny hairs detect how the fluid is moving around, and your brain matches that up with what you see. If you rotate your head, the fluid in the curved canal doesn’t rotate as quickly, so it flows around the canals because it hasn’t caught up yet. But if you spin for a while, this fluid starts to spin, too. It only takes a few seconds to catch up, and then the fluid in your ears is spinning steadily with the canals, matching the movement of its container. When you stop suddenly, the fluid doesn’t stop. Just like the raw egg, the container has stopped, but the fluid keeps going. So your inner ear is telling your brain that you’re moving, but your eyes are telling your brain that you’re not. That is when you feel dizzy, as your brain tries to work out what’s really going on. Eventually, the fluid in your inner ear does stop spinning, because its container has stopped. The dizziness fades away.
This is one of the reasons why pirouetting ballerinas keep facing in one direction as they spin, and then very quickly move their head all the way around to get back to the same direction as their body catches up. With this very quick stop–start motion, the fluid inside doesn’t get up to a steady spin, so the ballerina doesn’t feel disoriented when she stops.
There are two aspects to the conservation of angular momentum. The first is that something that isn’t spinning needs a push to get it going. It can’t just start spinning by itself. And the second is that something that is already spinning will keep spinning unless something pushes on it to make it stop. In our everyday lives, it’s often friction that provides the push to slow things down. So the spinning top eventually comes to a halt and the spinning coin slows down so much that it falls over. But in situations where there is no friction, things really will keep spinning indefinitely. That’s why the Earth has seasons.