Over and over again, I have the need to synchronize files between my home computer, my office laptop, and possibly other computers as well. I use git because a lot of these files are things that I might conceivably want to keep a history of, and also because git makes it easy to set up a work environment on another computer.

Project metadata files get in the way of this, though. These are the files that fancier editors use to keep track of things like which files you had open, what syntax highlighting schemes you were using, and so on. The file typically gets changed every time you open the editor. Now, most of that information is pretty trivial, and I typically don’t want to synchronize it between computers or keep track of its history. On the other hand, when I’m setting up on a new computer, it’s pretty handy to have a project file available to start with, so I’m reluctant to exclude the files from the archive entirely.

Today, I found a solution. After you clone your git repository, in the new work environment, run

git update-index --assume-unchanged <projectfile>

This sets a flag which …

News flash: I’ve been thinking. Shocking, I know… actually not at all. But, silly jokes aside, I’ve been thinking that it’s long past time I mention one of the “projects” that’s been taking up a fair amount of my time for the past few months: Physics Stack Exchange.

This week marks 100 days since the Stack Exchange network launched a Q&A site on physics. I’ve been involved essentially since the site first opened up, and in fact I’d been hoping for a site like this to take off ever since the first time the Stack Overflow team first proposed opening up the SO engine to other topics besides computer programming, back in 2009. If you look at traditional forum-based physics Q&A sites, like Physics Forums, a lot of topics turn into extended discussions as the participants try to hash out all the minor details. That discussion has its place, but most of the time, when you ask a question, you just want to get the right answer. And on Stack Exchange sites, that’s exactly what you get: the first thing listed after a question is the highest voted answer. So I …

It’s been far too long since I did a Mythbusters writeup, but I think it’s time to stop stalling and bring this series back. On this week’s episode, Adam and Jamie tested the myth of Archimedes’ heat ray for a third time — that has to be some kind of record — at the request of President Obama.

The gist of the myth is this: by focusing enough of the sun’s rays, using a large number of mirrors, on an enemy ship, the Greeks hoped to heat it up enough to make it catch on fire. So far (spoiler alert), there’s no evidence that this thing ever could have worked. All three of the Mythbusters’ tests have failed.

But I think I can shed some light (no pun intended) on why. As it happens, I taught a lab on thermal radiation transfer this week, and that (along with an interesting perspective on gravitationally baking a turkey) reminded me that it’s fairly straightforward to calculate, at least in a simple model, the amount of radiation it takes to heat something up to a particular temperature. It all stems from the Stefan-Boltzmann equation,

I’ve noticed that (many of) my intro-level physics students, who have typically just learned about vectors within the past year, have a hard time working with them in equations. For example, given an equation like \(\sum\vec{F} = m\vec{a}\), I’ll typically think of the force vectors in a geometrical representation, as arrows. Vector addition is done by starting each vector from the endpoint of the previous one in the sum, and scalar multiplication just means scaling the length of the arrow. But almost invariably, whenever I try to walk a student through this representation, they don’t get it. It seems like they’re really not prepared to accept the idea of doing math on things that they can’t plug into their calculators.

What I’ve found to work is this: I tell the students that whenever they see a vector equation, write one copy of it for each dimension. Newton’s second law, for example, becomes a set of three: \(\sum F_x = m a_x\), \(\sum F_y = m a_y\), and \(\sum F_z = m a_z\). That way, all they have to deal with are simple numerical equations. Sure, they miss out on understanding the true nature …

Mythbusters is back as of last week with new episodes! Although the season premeire about throwing dogs off the scent isn’t directly physics-related, I do have one issue to point out.

Tory, Kari, and Grant did an experiment in which they tried to hide some “contraband” from a dog by sticking it in a container with a strong-smelling material, like coffee or peanut butter. They hid the container somewhere in a 17,000 square-foot warehouse and set the dog loose to find it — which he always did. So the conclusion was that strong scents can’t distract a sniffer dog.

But what if the dog was actually following the distraction? I wouldn’t be surprised if the scent of e.g. peanut butter itself was a dead giveaway to the location of the container, even if did mask the smell of the fake drugs. It was probably the strongest smell in that warehouse. A better experiment would have entailed filling several containers with the smelly substance, distributing them around the building, and only hiding the fake contraband in one of them, then seeing if the dog identifies the right one.

Of course, they did basically that later in the …