Liam

4.3 Bernoulli’s Principle and Airplanes

by

Critical Questions:

  • How does an airplane stay up in the air?

You might think that today, more than a century after the Wright brothers flew that first airplane of theirs, most of the people who build and fly airplanes would agree on an explanation for how they stay up there. But just try walking into a room full of pilots and airplane engineers and asking them to explain this to you. The heated arguments and vicious character attacks that will result may well convince you never to step foot on an airplane again.

Upside Down Lynx by Captain Chaos on Flickr
Don’t even get me friggin’ STARTED on helicopters.

The reason why it’s so hard to pin down one easy explanation for the force that keeps an airplane up in the air – the ‘lift’, as it is called – is that it’s actually pretty complicated. Most simplified explanations of lift leave out some crucial details, or else they add in some incorrect ones.

The most common incorrect explanation of lift involves Bernoulli’s principle, which is interesting enough that we can spend some time getting to know about it before coming back to the problem of airplanes.

Read more

Another Way to Learn Physics Online: Landmarks in Physics

by

This week, as undergrads and high schoolers everywhere are plunging into the world of Introductory Physics, I wanted to share another great resource for learning this material. It’s a course on Udacity called Landmarks in Physics. Udacity features a free, easy-to-use interface for self-guided courses taught through a combination of YouTube videos, interactive quizzes, and … Read more

4.2 Buoyancy

by

Critical Questions:

  • Why do things float?
  • What is density?

I think it’s illegal to write anything about buoyancy without telling the story of Archimedes. So here goes.

Archimedes was a scholar who lived in Greece in the 200s BC. As the story goes, the king of Syracuse gave a local smith some gold and requested that it be turned into a crown. When the crown was finished, the king suspected that the smith had kept some of the gold for himself, replacing the stolen quantity with silver, which was much less valuable. Luckily for the king, there was a person in the kingdom who is recognized today as one of the greatest scientists, mathematicians, and inventors of all time. The king went to Archimedes and asked him to sort out the whole crown situation.

Archimedes Thoughtful by Domenico Fetti
Archimedes in badass chiaroscuro.

The crown was found to have the correct weight, but the smith could have put in just enough silver to match the weight of the missing gold. However, gold was known to be almost twice as dense as silver – in other words, one piece of gold would weigh about twice as much as an equal-sized piece of silver. So if the smith had indeed cheated, the crown would be bigger than expected – specifically, it would have a greater volume.

But nobody could figure out how to determine the volume of a shape as strange as an intricately-wrought crown. They could melt it and re-shape it into a nice cube (and just multiply length x width x height), but then the crown would be ruined.

Read more

4.1 Introduction to Fluid Mechanics

by

In 1903, at Kitty Hawk, North Carolina, Orville and Wilbur Wright flew their homemade airplane for 59 seconds, travelling in the air a distance of 260 meters. Their aircraft is generally recognized as the first heavier-than-air, powered vehicle, meaning that anything that flew before then was either a glider, a balloon, a blimp, or a bird.[1. Other possibilities include a bee, a bat, and a pterodactyl.]

Wright brothers' first flight

Although rocket technology is a whole different field, it’s nonetheless interesting to note that it was only 66 years later – less than a human lifetime – that the first human beings walked on the surface of the moon. Somebody could have witnessed, as a child, the first airplane to ever take to the sky, and then grown up and gotten on a commercial airliner to Florida to watch Neil Armstrong and Buzz Aldrin take off in a Saturn V rocket.

Read more

Video of the Curiosity Landing

by

(Edit: I’ve replaced the original video with a much better one. The original can still be viewed here, but I don’t know why you want to.) Today being Wednesday, we would normally have carried on with the next chapter of our little textbook. But it’s still summer, and humanity’s got this new car-sized robot on … Read more

Curiosity Has Arrived!

by

The new Mars rover, Curiosity, has successfully landed on the surface of the red planet. This is a pretty big achievement. To understand why it’s so crazy that NASA was actually able to pull this off, check out this video from an earlier post.

3.7 Black Holes

by

Critical Questions:

  • What is a black hole?
  • What happens near a black hole?
  • How do we know they exist?
  • What about wormholes?

In order to explain what black holes are like, I’d like to relate to you the following parable.

A bunch of physicists are up late one night. They’ve been at a party. Most of the guests have left by now, nobody’s noticed that the music has stopped, and the air smells only faintly of sweat and a remarkable range of mind-altering substances. Somebody is passed out on the couch, but no one seems to know who it is. During a lull in the mumbled conversation, one of them happens to look up through a skylight and see the stars. She begins to think about how far that light has travelled to reach Earth, and about why it didn’t just stay where it is. She says so, out loud. One person mumbles in agreement. Another person suggests that maybe some stars don’t allow their light to leave at all. This sparks a two-hour long discussion that includes a whole complement of theories related to this kind of dark star, with names like the no-hair theorem and the information loss paradox. Before the last of them finally falls asleep, he mutters, “Black hole. We can call it a black hole.”

Chemistry Dog
Or maybe it was this guy.

To the best of my knowledge, this is not a true story. But if you learn enough about black holes, you might be tempted to think that this is how people thought up the idea.

Read more

3.6 General Relativity

by

Critical Questions:

  • What is General Relativity?
  • How do we know that general relativity is correct?

Ok, if you’ve already read the previous post, we can now dive into the awesomeness of General Relativity.

I’ve already said that Isaac Newton was bothered by his own theory of gravity because it seemed to involve things affecting each other through empty space, without ever coming into contact. In fact, nobody felt comfortable with the idea of a force that acted at a distance: it seemed too much like fantasy and not enough like solid science.

Isaac Newton
It’s probably what he was frowning about in this portrait.

The only problem was that this theory worked very, very well. It explained almost everything related to gravity, most notably the motions of all of the planets and stars in space. It was even used to successfully predict the existence of the planet Neptune based on the motions of Uranus (albeit almost 200 years later), and verified predictions are the true test of any scientific theory. But Newton wrote the following in a letter six years after publishing Principia:

“That gravity should be innate, inherent and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me a great absurdity, and I believe that no man who has in philosophical matters a competent faculty of thinking, can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial I have left to the consideration of my reader.”

This was a daunting challenge, and one which most people judiciously ignored until Albert Einstein began working on General Relativity in 1907.

Read more

3.5 Things to Know Before General Relativity

by

Critical Questions:

  • What can we say about General Relativity without using math?
  • What does ‘relativity’ mean?

Before I even say one word about General Relativity, I feel obliged to issue a stern warning: prepare to be frustrated.

You see, most of the rest of the physics you’ll see on this site originated within one or two hundred years of Isaac Newton and the invention of calculus. This means that although there is some quite difficult mathematics behind it, most of it is based on direct observation and can be understood from a conceptual standpoint without worrying too much about the math.

But by the time Einstein came along, the field of mathematics had made some significant progress. All of the physics theories loosely called ‘Modern Physics’ (the Theories of Special and General Relativity and Quantum Mechanics being the main elements) involve lots and lots of extremely difficult math, and General Relativity is no exception. Read almost anything on the subject, and you’ll only need to encounter a few words like ‘semi-Riemannian metric’, ‘Lorentz invariance’, or ‘tensor fields’ before realizing that a true understanding of this stuff requires a small library of books, a PhD, and godlike determination.

Make Child Smart Math English
Or just call this number.

So when it comes to modern physics, if you want to avoid the math, you’ll be limiting yourself to hearing only the consequences of the theory rather than getting satisfying arguments for why it must be true.

But with that said, the consequences are so important and yet so eye-gougingly bizarre that they deserve some mention.

Read more

Real Answers to Ridiculous Questions

by

Randall Munroe, author of the ever-popular geeky stickman webcomic XKCD, is also a physics & math nerd. So he gets a lot of weird hypothetical questions from his readers. Now he’s starting to answer them, and it’s pretty entertaining. Here’s a snippet from his first question, What would happen if you tried to hit a … Read more