There are Known Knowns; There are Things we Know we Know

March 19th, 2016

And Little Lambs Eat Ivy

I don’t get into politics much these days, and no one is interested in religion, so I do not attract trolls like I used to. They used to take up a good percentage of my waking life.

A wise man (clearly not me) once said that it is not absolutely necessary to gainsay everything you disagree with on the Internet. I wish I had thought about that in 2002, before I started blogging. I would never have gotten involved in whack-a-troll, and I would not have treated well-intentioned commenters like trolls, which I am sure I have done. Worse than that, I’m sure I have corrected people who were right when I was wrong.

Today I showed up to see if anything was going on here, and I saw a somewhat trolly comment in the approval bin. I don’t have it in front of me (it was accidentally deleted in a process which would have made Fusilli Jerry proud), but the first line was something about me being confused–not a sign of warmth or good intentions–and the commenter tried to correct me about the concept of escape velocity. He wrote some really strange things which just were not correct. They were so wrong I did not even understand them. It’s like the famous quip from physicist Wolfgang Pauli, who once disagreed with someone so intensely, he said, “That’s not even wrong.”

I hold comments for approval for good reasons, and one reason is to avoid getting into comment wars with people who are not able to understand the topic. There is nothing worse than arguing with a person who doesn’t know what’s going on.

How does he know when he loses?

I guess they just figure they’ve won when you stop responding.

I am not a real physicist, if a physicist is a person who can sit down with no preparation and do physics well enough to be paid for it. On the other hand, I am a real physicist by the definition of the people who sell magazines to former physics students! If you belonged to the national physics honor society at one time, and now you’re a realtor who can barely integrate 1/x^2, but you can pay for a subscription to Physics Today, you, my friend, are a physicist.

I studied for several years, I got a degree, and I went to grad school in physics for a year and a half, at the end of which my brain turned into a pork rind, and I had to quit. I remember little bits of what I learned. Don’t ask me to write out the Laplacian for spherical coordinates, and don’t even talk to me about quantum mechanics, but I know a couple of things.

I had to look “Laplacian” up just now to make sure I didn’t really mean “Lagrangian.”

Anyway, I didn’t pull my terrific explanation of escape velocity out of a bong. Here is how it works, and this time I actually looked at someone else’s web page to make sure I didn’t say anything crazy. Does that make me a plagiarist? No, because it would be like plagiarizing addition. And it agreed with what I said to begin with.

There are two kinds of energy in classical mechanics: kinetic and potential. To have kinetic energy, you have to be moving. To have potential energy, you have to be in a situation where something or other has the potential to make you move.

If you’re falling from a building, you have kinetic energy, which will be transferred to whatever you land on. If you’re on top of a building, you have potential energy, because if you fall off, you will rapidly start to move.

If you’re a rock in a slingshot, pulled back all the way, you have potential energy. If you’re released and start flying through the air, you have kinetic energy.

The energy in an object can go back and forth between kinetic and potential energy.

Think of a bouncing ball.

As the ball leaves the ground, it’s moving at its highest possible speed. It’s going as fast as it will ever go, because as it moves upward, gravity will slow it down. All of its energy is kinetic.

At the top of the bounce, the ball is not moving. It has no kinetic energy. All of its energy is potential energy. As it falls again, that energy will turn back into kinetic energy.

Balls don’t bounce forever, because energy is lost to air resistance and the force it takes to flex the ball and radio waves and God knows what else. In the real world, I mean. In physics books, balls bounce for eternity, because the problems take place in ideal worlds that don’t exist.

Are you with me?

Imagine an object on the surface of the earth. There is no air resistance, because OUR [fictional] earth is ideal. There is no friction. There is only gravity. There isn’t even any sales tax.

If you shoot the ball upward, it will return to earth, because it will be slowed down by gravity. In reality, it’s falling from the instant it leaves the ground. It’s falling AND rising. By “falling,” I mean it’s accelerating downward. If you don’t understand that, don’t worry about it. No one cares.

Satellites in orbit are always falling! They’re going sideways so fast it cancels out the fall!

Sorry. That just popped out. Ignore it.

As our ball goes up, it loses kinetic energy because it slows down. It gains potential energy. That potential energy is related to the distance the ball rises. It increases on the way up.

If you do the correct definite integral with the correct endpoints, you find that if the ball rises all the way to infinity, the amount of potential energy it gains will not be infinite. It has a cap. The potential energy a ball can gain as it leaves a planet is limited.

If, when you shoot the ball upward, you give it enough speed to give it kinetic energy greater than the total possible potential energy, you have given the ball what is known as escape velocity. It escapes from the planet’s pull. The planet can’t pull it back. It keeps going forever.

This only works on ideal planets, which are rare, although Bernie Sanders says he knows how to create one using other people’s money. It didn’t work too good in Cuba and North Korea, but we can always hope.

If this kind of thing actually interests you (stop laughing), you can find escape velocity explained, with integrals, all over the Internet. I’ll link to a page which lays it all out in a couple of paragraphs.

Gravitational Potential Energy, Explained

An interesting thing to observe, if you look at the integral, is that the smaller the radius of your planet is (for a given mass), the harder it is to leave. This is why it’s pretty hard to come home from a vacation on a black hole. Lots of mass means lots of gravity, and a small radius means it’s sort of crammed into a tiny area, so it’s strong.

What I wrote is not advanced physics. I guess it would be in the first semester of university physics, which is the low-level class your doctor took. It’s not controversial or complicated.

Now, if you ask Bernie Sanders what he thinks, you may get a different answer. I can’t be responsible for his interesting notions.

I don’t plan to start publishing trolly-looking comments. Life is too short. My advice to irritated commenters is this: put them on your own blog, where they will actually see the light of day. I delete them permanently.

I apologize if the comment I dumped was not intended to be a troll scat, but really, I could not see any way to make it mesh with reality, so I assumed the worst. Another bad Internet habit.

Maybe I misunderstood the commenter’s jargon, but in any case, it’s clear that what I wrote initially was correct. If anyone else wants to come by and challenge trivial results of undergraduate physics problems, please don’t.

4 Responses to “There are Known Knowns; There are Things we Know we Know”

  1. John P Says:

    Good stuff! I took that Physics 101 class freshman year and that was about it for my science career. My brother, on the other hand, went all the way to his PhD in Physics and now works on research involving the super-collider in Switzerland, while employed by U Cal Berkeley.

    As you could imagine, he’s as hard-boiled an atheist and leftist as is possible. Sweet guy, but seduced by his intellect, and believes his science acumen translates into expertise in all sorts of areas outside of his field. He’s in his mid-50’s so is pretty hardened in his beliefs…which work very well for him in this world.

    As you could also imagine…we don’t talk faith or politics! His “comments section” is one I don’t allow in my life, and vice versa, so we get along fine. Who knows what the Holy Spirit has planned for him!

  2. Steve H. Says:

    I keep trying to figure out why someone would come here and pretend to know more about things than I do, when he apparently knows nothing at all about them. The weirdest part is that the comment was condescending.

    My mistake is that I look for logic in people’s actions.

    He said something about air resistance not being important in rocket launches. Can you imagine the backward force on a Winnebago-sized rocket moving at several hundred miles an hour?

    It reminds me of something a motorcycle rider at Bonneville said: when you get above a certain speed, the air is like yogurt.

  3. Ruth H Says:

    In reply to the John P comment:
    I call that PhD syndrome, it also happens to those who do not have PhD. You know lawyers, dentists, MD’s, school principals, people like that. I’ve been married to a Phd for almost 58 years now and although he is definitely not an atheist, he does, or did, think he was capable of doing almost anything. And he did, he always did our car repairs, actually wired and plumbed the house we built in 1997, (his choice; the I can do this syndrome) has always done our taxes, more of the I can do this, thing, and numerous other things. But he has reached 80 and realizes he cant do that stuff anymore, and it hurts. I on the other hand am the computer nerd and I can still do that stuff. YAY ME. Do I have a little bit of that syndrome? Why yes, I think I do.

  4. James the lesser Says:

    I see that a lot too (worked on LHC/CMS for a few years myself). Not all of us are atheists, though.

    Remember the math problems you had to solve? Prove that for X, Y is true. And you do it in half an hour. Once you know it’s true, the path gets a lot easier. When we crank through as exercises things that took smarter people than us years to figure out, I figure it can’t help but swell our heads a little bit: we can do anything!