Archive for the 'Physics' Category

Cousin to an Elephant Whose Mass May Be Ignored

Figuring otu when ignoring air resistance is a bad approximation.

When Does the Air Resistance Force Make a Difference?

With only the gravitational force, the object has a constant acceleration and the motion is fairly simple to model.

However, objects on the surface of the Earth usually have an air resistance force on them also. When can we ignore this extra force and when is it important?

Word to Your Mother, and Everyone Else

Science Word of the Day: Kleptothermy

Know When to Fold Them

Don’t bet on the failure of relativity

[T]his isn’t a modest proposal at all, but (to use the unkind phrasing usually attributed to unkind physicist Wolfgang Pauli) it isn’t even wrong. His modification to the laws of physics are too large, with implications he avoids by not dealing with them. Instead, he focuses on a few small aspects of relativity: the loss of simultaneity between moving frames of reference and the time-dilation effect measured by two observers moving rapidly with respect to each other.

Sunshine, Lollipops And Rainbows. OK, Just Rainbows

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Curious About Curies

Radiation from Fukushima is reaching the West Coast — but you don’t need to freak out

In fact, what’s truly amazing about the work is that scientists are able to actually measure these very low levels of radiation at all — as well as to chemically fingerprint them and thereby prove that certain radioisotopes of the chemical element Cesium, which arise as a by-product of nuclear fission, actually arrived off of North American waters after traveling all the way from Fukushima.

The big non-news is that the radiation levels are small — hence the amazement at being able to measure them. But say “radiation” and some fraction of the population freaks right the hell out. So that’s the take-home message, even if there are a few subtle things missing in the story.

The activity (how radioactive a sample is, measured these days in Becquerels, or number of decays per second) is not the whole story, because not all radiation damages the body the same amount, and it matters greatly if the contamination accumulates in the body or not. Their example is swimming in the ocean, but I think people might also be concerned about eating fish, who would effectively be filtering out and accumulating radioactive material. Are they twice as radioactive as the water? Ten times? Is the internal dose more damaging than an external dose (that’s true of alpha and beta radiation)? Cs-137 is a beta emitter, and in humans it accumulates in the body, with a biological half-life of 70 days. So I imagine it accumulates in fish, as well. But with such low starting levels, probably not anything to worry about.

Another nit is with the picture down near the end, the “helpful figure from the Woods Hole Oceanographic Institution” is somewhat less helpful than it could be. First of all, it doesn’t have the Fukushima incident on it as a comparison. I’ve seen a few links, and their estimates vary by around an order of magnitude, but it’s between ~4 million and ~40 million curies of Iodine and Cesium into the atmosphere, and one link had an additional amount going into the water of half of the atmospheric discharge. So there’s your comparison. (A curie is 3.7 x 10^10 decays per second, as that was Marie Curie’s favorite number is that activity of a gram of Ra-226. But it’s a huge number and not normally useful for everyday discussion — you are usually talking about picocuries or nanocuries, or something like that.)

Another nit is that comparing activities between different isotopes is a tad dicey, for a couple of reasons. The activity is a rate, not the total amount of potential dose. 1 curie of a contaminant that has a half-life of a day is very different than a curie of something else that has a half-life of a million years — in 10 days, the one-day half-life isotope is down to 0.1% of the activity (a millicurie), while the other is still basically the same. When you look at the Uranium and Potassium numbers on that infographic, keep in mind that K-40 has a half-life of 1.3 Billion years, and Uranium-238 is 4.5 Billion years. Their respective activities aren’t going to perceptibly change in the next 30 years, while any contaminant Cs-131 will drop in half. Further, Uranium is an alpha decayer, which is the most damaging internal source (though if it’s external is pretty harmless — an alpha won’t penetrate your clothes or through the dead layer of skin on your body). Uranium also decays through a chain of radioactive daughters, which have their own decays to contribute. So the activity doesn’t tell the whole story — the reason the ocean’s Uranium and Potassium aren’t a big issue is that the ocean is huge. Fukushima’s release was concentrated, but owing to time and dilution, it’s not a problem for those of us in the US.

Idle Engines are the Devil's Playthings

The biggest winter energy myth: That you need to idle your car before driving

During the 1980s and into the early 1990s, however, the auto industry did away with carburetors in favor of electronic fuel injection, which uses sensors to supply fuel to the engine and get the right air and fuel mix. This makes the problem of warming up the car before driving irrelevant, because the sensors monitor and adjust to temperature conditions.

Another big myth, at least based on how I’ve seen people drive, is that it’s a good idea to spin your tires to get traction. But since the coefficient of sliding friction is lower than that for static friction, once the slipping begins, you’ve lost. You should ease off the gas and start over.

Tiny Bubbles

My trip home for Christmas gave me my first shot at using my thermal IR camera in quasi-freezing conditions — it ended up being warmer than usual, but still dropped to freezing, or close to it, when the sun was down. I did a survey of my mom’s and a neighbor’s house (confirming for the neighbor that the section of roof where the snow melts fastest is warmer than the rest). Not surprisingly, the warmest parts were the windows, and we have lots of windows. The largest ones are now double- or triple-pane, so it used to be be worse.

Several basement windows, though, are still single-pane. The whole idea of insulation is to trap dead air, i.e. it won’t convect and it also conducts poorly, so I tried a little experiment: putting some bubble wrap up against the window, wrapped in some thin packing foam. I left a slice of it uncovered as a control.

You can see that the left side is noticeably warmer than the insulated part. I went online to see if anyone made insulation made specifically for glass, and ran across a site that had done pretty much what I did — just using two layers of large-bubble packing material so I went out and bought some. The basement windows are now no warmer than the rest of the cinder-block walls. The bubble wrap is translucent, so most of light still gets through.

I used the last of the wrap on some windows on the north side of the house, so I’m not blocking any incoming light that helps heat during the daytime. In my test, I blocked off one section

The window I tested is the one on the left. It’s slightly cooler than the rest. I used one of the other FLIR programs to confirm it’s about 1 ºF cooler than its neighbors (those pictures don’t get saved to the camera roll, though. Not sure why). These are multi-pane windows so the effect isn’t as dramatic, but every little bit helps, so I ended up covering up a few more windows to use up the bubble wrap I had bought. They’re in a part of the room that’s partly blocked anyway, so it doesn’t really impede looking out. This could be re-used from year to year (the basement windows probably left up permanently) and only took a few minutes to do.

A Hard Day's Night

Correction: Tonight will not be the longest night in the history of Earth

The article below said that, due to the rotation of the Earth gradually slowing down over time, this winter solstice would feature the longest night ever.

I got this wrong. The Earth’s rotation is gradually slowing on an extremely long timescale, but on a shorter year-to-year basis, geologic factors can alter the speed as well.

Right basic idea— the rotation rate is slowing as a long-term trend — but wrong execution.

Being Disagreeable

Scientific method: Defend the integrity of physics (via zapperz)

Faced with difficulties in applying fundamental theories to the observed Universe, some researchers called for a change in how theoretical physics is done. They began to argue — explicitly — that if a theory is sufficiently elegant and explanatory, it need not be tested experimentally, breaking with centuries of philosophical tradition of defining scientific knowledge as empirical. We disagree. As the philosopher of science Karl Popper argued: a theory must be falsifiable to be scientific.

Wow. Add me to the “disagree” list. This seems like it would be a huge step backward — like zapperz said, this is more like religion. We would have dogma, not science. It harkens back to the philosophy of old, before modern science. Heavier balls fall faster than lighter ones — that’s pretty elegant. Why bother testing it? Things move owing to their inherent nature. There are four lights elements. The universe is in steady state. Galilean transforms and Euclidean space are pretty elegant, too. Too bad all of these are wrong, to varying degrees, because they disagree with nature — and that’s the requirement we have for all of science: it has to describe what actually happens in nature, and because we can be fooled by a great explanation, we have to know we’re getting it right. The article also points out the danger of a model that’s too vague, so that any result can be explained. Not a lot of scientific value in that.

What is the measure of elegance, anyway? Without falsifiability, there’s no way to know if you’re wrong. What if you have two competing theories — do they wrestle for it? Talent competition?

Do we really want to go there? I don’t think so.

Are You High, Noon?

High Noon

If we consider “noon” to be the time at which the sun is highest in the sky then the time between successive noons is not quite 24 hours. Relative to our clock, our sundial will seem to run a bit fast on some days, and a bit slow on others. Because of this, if we note the position of the Sun at clock noon over the course of a year, it will mark out a figure-8 pattern known as an analemma.

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