Missing explanations

Posted by – April 10, 2017

There are some things that people think they understand, or assume to be straightforward to understand, but are (apparently) impossible or very difficult to understand properly. I think about explaining things a lot these days due to being a dad, and I always did like explanations, but all of these stump me to some extent.

An easy one to start. Imagine that you’re standing next to a bicycle with handlebar brakes. You’re holding the bicycle up and can roll it forward or backwards. Now, imagine you engage the back wheel brake. Can you now move the bicycle backwards? What about forwards? Now, engage the front wheel brake. Can you move the bicycle backwards or forwards?

What happens is that with the back wheel brake engaged, you can move the bicycle forwards, with the front wheel rolling freely and the back wheel sliding with some friction, but not that much. But you absolutely cannot move the bicycle backwards. The front wheel will lift up, and the bicycle rests on the locked back wheel. Now, I’m not saying that I don’t understand what’s happening here. But it’s somehow awkward to put it in words, and you can easily give an explanation that is missing the point, or begging the question. Especially at a first try. Go ahead and try!

Next, how does a bicycle stay upright when you’re riding it? A lot of resources will tell you that it has to do with the gyroscopic action of the wheels spinning, but the force from that is not big enough, and besides, you can stay up even at quite low speeds. Though it does give a piece of the puzzle. In fact there’s no single explanation, just lots of little pieces. Most people can’t keep the bicycle stable without using their hands on the handlebars to provide feedback to the front wheel, but other people don’t need to do that. They rely on adjusting their centre of gravity over the bicycle, plus other effects. There isn’t really any good explanation, even if you’re an engineer. On the other hand, it’s not like the physics aren’t understood – it can be simulated in a computer just fine.

A similar case is the wing of an aeroplane. The first explanation I heard was that at the leading edge of the wing the airflow separates, some going under the wing and some over the wing. Because the trip over the wing is longer, the air gets thinner, so the pressure is lower above the wing. This low pressure then sucks the wing upwards, and that’s what causes lift. But this explanation raises many questions. Why does the air take the same amount of time going over and under the wing? Why doesn’t it get deflected and flow away? How can aeroplanes fly upside down? This is the “Bernouilli effect” explanation, and it is not sufficient, even with combined with additional effects (eg. the Coandă effect causing air to stick to the contours of the wing, like the way water flowing from a faucet will be “bended” by the side of a drinking glass put in its path).

The only thing I can really honestly say to a child about an aeroplane’s wing is that the wing pushes and sucks air down, both from above and below the wing. At least this is what smoke in wind tunnels shows. I can’t properly explain why. And that the angle at which the wing meets the air is important – only at some angles does a wing generate lift. The wing doesn’t even need to be curved, though it does help. If you fly upside down, you probably have to angle the winds differently from right-side-up flying. If you slice your hand through water in a pool you can get an idea of how the wing needs to be positioned.

And I think something like the Coandă effect does have *something* to do with it – in fact, I believe an aeroplane could not fly if air had zero viscosity, but I’m not even 100% sure about that. Again, the wing can be modeled mathematically just fine, it’s just difficult to explain in words.

Finally, a question that most people (me included) don’t even think to ask even when they easily could have – and when I first heard it, I was very confused, because I first thought I understood the elementary physics, but had in fact encountered a quantum effect. Namely, if electrons are negatively charged and protons are positively charged, why don’t the electrons fall into the nucleus? Well, isn’t it like planets and the sun, where the electrons are attracted to the nucleus but moving fast enough to keep from falling in and they stay in orbit? But an accelerating charged particle would lose energy to radiation, and when you calculate it classically, the electrons really should fall into the nucleus. Oh, I know, this is that quantum thing, where the electrons are only allowed to have certain quantized amounts of energy, so they won’t radiate anything away and just keep going around in orbit. This is the “Bohr model” of an atom, but it’s also incorrect, predicting observations only for hydrogen atoms for which it was designed.

In the end, there’s a lot you can say about the quantum nature of an atom – like that the electrons don’t have a well defined position and speed anyway, so they can never be on a trajectory falling into the nucleus, or any other point – but nothing to really satisfy our classically conditioned minds. Again the only answer is in the mathematics, but it doesn’t explain.

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