Editore"s Note
Tilting at Windmills

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November 1, 2007
By: Kevin Drum

GOOD READING....I've fallen woefully behind on articles and blog posts I want to link to. Rather than just delete them en masse from my email inbox, though, here they are in quickie linkdump form:

  • This week National Journal rated the healthcare plans from the main presidential candidates. Basically, they created 13 categories, or goals, and gave each candidate a score of 1-10 for each goal. The Democrats ended up being bunched pretty tightly: Hillary Clinton scored 86, John Edwards scored 83, and Barack Obama scored 81. The Republican scores were so pitiful I didn't bother adding them up. Details here.

    UPDATE: As Matt points out, the basic breakdown here is that Democrats do well on goals associated with providing better healthcare to more people, while the Republicans tend to do well only on categories related to spending less money. In terms of actually accomplishing anything, the Republican plans pretty much suck.

  • In "Spies Around the Watercooler," Laura Rozen writes about the unique problems faced by women who work for the CIA. Turns out it goes beyond just Valerie Plame.

  • Jeb Koogler notes that al-Qaeda in Iraq is getting pretty good at hearts-and-minds counterinsurgency.

  • Over half of all public school children in the South now come from families poor enough to qualify for school lunch assistance. And no, it's not because all the rich ones are leaving the public school system for private schools. There are just more families who don't have much money these days.

  • What if torture works? Does that make it OK?

  • Ilan Goldenberg links to a new GAO report that says our efforts in Iraq "lack strategies with clear purpose, scope, roles, and performance measures."

  • Salon has a long piece tonight about the Bush administration's war on whistleblowers. Don't let the sitepass stop you from reading it; it only takes a few seconds to get through it.

  • Stories like this make me fear for the future of the country. The punchline is at the end, of course.

  • Attention physics nerds: the New York Times says a plane flying through a crosswind travels faster than it would in clear skies. I say that's bogus, assuming you actually want to get to your destination. Anyone care to referee this problem? Your competition is a bunch of precocious ninth graders.

Kevin Drum 12:24 AM Permalink | Trackbacks | Comments (65)

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The crosswind question isn't phrased in a practical manner. But, consider a plane whose airspeed is 0 but encounters a 10 mph crosswind. (And by airspeed I mean the component of airspeed along the lengthwise axis of the plane.)

But it's a mostly useless question. A more useful question is, if you want to head north and the wind is blowing due west, does it take longer to get to where you want to go than if there was no wind at all.

Posted by: Sam Shen on November 1, 2007 at 12:41 AM | PERMALINK

It's the same principle as a sailboat, isn't it?

Posted by: SP on November 1, 2007 at 12:46 AM | PERMALINK

The sex offender story is disturbing because, like the Amber Alert system it's another way to make us buy a growing police state "for the children."

Last summer there was a spate of child kidnapping stories, even though the number of actual child kidnappings was down. The RFID tags for pets they're pushing now? Next they'll be for kids, for their own safety. Within a generation the total clampdown will be in place.

But wasn't this freedom shit fun for a while?

Posted by: thersites on November 1, 2007 at 12:56 AM | PERMALINK

The exact phrasing, like in many physics questions, matters immensely.

It is only talking about absolute land-speed, and the direction of the crosswind is relative to the direction of motion, not any specific destination.

So yes - plane travels faster - but that won't guarantee you get to the destination earlier.

Posted by: MobiusKlein on November 1, 2007 at 1:01 AM | PERMALINK

Sailboats use a plane in another media to translate force of the wind into motion. But in essence, it is similar.

I'm trying to remember my aeronautic engineering course from fifteen years ago... Oy.

Posted by: Crissa on November 1, 2007 at 1:26 AM | PERMALINK

If the pilot keeps heading in the exact same direction he would have if there had been no crosswind, then the plane will be carried sidewise by the crosswind and its speed in the direction it is actually moving will be greater. (It will also veer away from its destination).

But if the pilot corrects for the headwind (as I presume pilots do) and turns slightly into it, then the plane will move in the exact direction it would have without the wind, and, presumably, at the same speed.

Posted by: JS on November 1, 2007 at 1:28 AM | PERMALINK

Of course, if the pilot adjusts the direction then it's no longer a 90 degree crosswind. Or is it? Depends on your definition of a 90 degree crosswind (angle measured w.r.t to the fuselage or the actual plane motion?)

Posted by: JS on November 1, 2007 at 1:37 AM | PERMALINK

I see everyone else is tackling the airplane crosswind = sailboat thing too. Lemme throw my $.02.

Sails look just like (are just like) airplane wings when seen end-on, like the Nike shoe logo.

Sailboats move forward because air travelling over the curved surface creates high pressure in the windward side and low pressure on the lee side. In truth, the boat should just move in the same direction as the wind flow, but the keel prevents that. The equivalent of a boat keel is that big airplane tail.

Sailboats have no propulsion beyond the wind flow. Now it shouldn't be a big leap of intuition to see that airplanes would also get some forward impulse from a crosswind, in addition to that provided by their engines.

As for your other questions:

Torture DOESN'T work, so your question is moot. Stop enabling Republican frames.

What are you getting at with the 'fear for the future' link? Curfews for child molesters on Halloween is a bad thing?

Posted by: anonymous on November 1, 2007 at 1:38 AM | PERMALINK

The crosswind thing is bogus. When you turn the plane into the wind to compensate for it, that reduces the forward component of your thrust vector by an amount proportional to the cosine of the angle you had to turn.

Posted by: Evan on November 1, 2007 at 1:42 AM | PERMALINK

P.S. I didn't mention that the equivalent vector of 'forward' for a boat is 'lift' on a plane. Crosswinds are free lift. Less energy spent toward maintaining altitude means more for going forward.

Posted by: anonymous on November 1, 2007 at 1:46 AM | PERMALINK

Speaking as a pilot and flight instructor This has practical real-lfe application. If there is a wind 90 degrees to the desired course then the ground speed will be decreased. This is beacuse in order to keep the plane on the desired course line that course line must be the result of two vectors, one that heads in the direction of the wind to keep on course, the other being the wind. Since some of the force propelling the plane must be vectored away from the course line there is less force along that line. If you draw a vector diagram the course line becomes shorter.

If however you accept the drift that the wind brings then the force propeling the aircraft is directly along the course line and the wind vector is additive. The resultant vector is longer, the speed greater. You would be saying the resultant vector is the course line now. But that pts the wind compnenet at a slight tailwind to that new course line.

The rule I teach is that if the wind is not at least somewhat to your back it will slow you down.

Here's anther one that gets most people. If you fly a course that is directly with the wind at your back and then return on the reciprocal of that course directly into the wind it will take you longer to do the round trip with the wind blowing for the whole trip than it would if the wind was calm. Intuition tells you you would make up whatever speed was lost going into the wind on the downwind trip but no. Do the math. To make it easy assume a 100 mile trip each way with a 100 mph plane and a 50 mph wind. Without wind it takes 2 hours round trip. With wind it takes two hours just to go one way against the wind.

Posted by: nameless bob on November 1, 2007 at 1:53 AM | PERMALINK

Hell, I know enough to defer to a guy who flies for a living.

Posted by: anonymous on November 1, 2007 at 1:56 AM | PERMALINK

The whole plane argument is predicated on having a swept wing, of course. Without a swept wing there would be no component of lift from a 90degree to flight path wind. Since most people are familiar with the wings on jets which are all swept, angled less than 90 degrees from the rear of the plane I guess most people take this form granted. Since this will only affect one side of the plane it will not be a symmetric force like those of a head or tail wind and thus not additive or subtractive from the airspeed.

Posted by: RC on November 1, 2007 at 1:59 AM | PERMALINK

Crosswinds are free lift. Less energy spent toward maintaining altitude means more for going forward.

It's not clear that a wind at 90 degrees to a plane's fuselage axis will create positive lift. My guess is the net lift effect will be different for different planes -- and could be negative as well as positive.

If the pilot turns slightly into the headwind, then the plane will definitely get some extra lift from that. But it will also use up more energy fighting the headwind. Not clear what the net will be, but I would guess it needs more fuel to travel at the same speed.

Posted by: JS on November 1, 2007 at 2:01 AM | PERMALINK

That's the point, you don't turn the aircraft into the wind. That'd be dumb, and fighting the wind.

An aircraft has wings, which produce lift via drag. It also has some sort of propulsion. It is traveling in three dimensions. The drag of creating lift slows the rate at which the aircraft's propulsion can move it forward.

An aircraft is always trying not to fall toward the earth. But like a sail tacking into the wind, but more like 'falling' into the wind. So instead of fighting the wind, the aircraft tilts slightly into the wind, so that some of the lift that would have been created by the propulsion is now created by the wind. That allows the propulsion to be used more for forward motion instead of vertical motion.

Posted by: Crissa on November 1, 2007 at 2:09 AM | PERMALINK

What about the river and the life preserver? I say go for the one behind you; as you fall into the river, it will take some time for your speed to equalize to the river's speed, while life preservers are either already moving at the river's speed, or would equalize much faster than your body.

But from my experience with word problems in math class, I think you're intended to assume that your speed has already equalized to the river's, so you're supposed to answer that it doesn't make a difference which way you swim. I always found these kind of "gotcha" word problems stupid, because if you don't specify all the relevant facts they aren't quite answerable, while if you do specify all the relevant facts, it gives the game away.

Posted by: brooksfoe on November 1, 2007 at 2:09 AM | PERMALINK

The ground velocity is the vector sum of the wind velocity and the air velocity, so of course the ground speed is faster in a cross wind.

Posted by: capitalistimperialistpig on November 1, 2007 at 2:11 AM | PERMALINK

...Umm, of course, there is an amount of weight to lift issue you have. If you have alot of lift to your weight - like a small plane - you won't be able to fall into the wind fast enough to make it up.

Light planes are always fighting the wind.

But a large plane which has less lift to its weight, will be able to make more out of the wind.

Gliders, for instance, are always falling into the wind to create lift. That's why it works at all. Of course, they do not have propulsion, and therefore can't go faster.

Posted by: Crissa on November 1, 2007 at 2:15 AM | PERMALINK

Excellent to link on the misguided sex offender story, Kevin.

Everybody inflates "stranger danger" way out of proportion, and society still refuses to talk about incestuous parents, grandparents, aunts, uncles and even older siblings.

Posted by: SocraticGadfly on November 1, 2007 at 2:20 AM | PERMALINK

You should do the backstroke towards the upstream life preserver, because when you're being swept downstream it's safest to go feet-first on your back.

Posted by: anon on November 1, 2007 at 3:19 AM | PERMALINK

The article says the students explain their answer using Newtonian physics and diagrams with triangles. Lift and drag are beyond Newton, so obviously they are explaining it by summing two vectors: the forward motion vector that is unchanged by the crosswind and the side motion vector due to the crosswind. (A diagram of the sum of two perpendicular vectors is a right triangle with the two vectors on the sides and the sum on the hypotenuse.)

Since the sum is longer than just the forward motion vector with no crosswind, they conclude that your ground speed is higher.

As Kevin pointed out, who cares if you speed is higher if you are not going the direction you want?

Posted by: treetop on November 1, 2007 at 3:42 AM | PERMALINK

The airplane/wind problem is simply an extension of the life preserver problem, adding a different frame of reference for consideration.

A properly trimmed aircraft, while in flight, does not care what the wind is doing. The speed through the air (indicated airspeed) will be the same no matter the direction of flight.

Once you add the consideration of groundspeed, however, then windspeed and direction become important. As pointed out, the constraint of 90 degrees needs to be further refined, 90 degrees to the flight path or 90 degrees to a course referred to the ground.

Consider what happens when you propel a boat to cross from one side of a (flowing) river to another.

If you do not care where you end up, then the flowing river will have no effect on your arrival time (assuming parallel sides and constant speed - in this case, wind 90 degrees to ones path through the air)

If you do care where you end up (say directly across from where you started), then you must point your bow into the current, effectively dividing your speed between crossing the river and compensating for the current (wind 90 degrees to ones desired ground track). If will take longer, depending on the speed of the current.

Angled wings have nothing to do with the exercise, unless one is not trimmed properly, but then, you have other problems.

Posted by: IntelVet on November 1, 2007 at 7:05 AM | PERMALINK

One thing to watch out for in grade-school treatments of how airplanes fly is the 'Bernoulli Principle' explanation for lift-- easy to understand and, unfortunately, bogus. Try, e.g., explaining how an airplane can fly upside-down. See Google for details.

Posted by: MattF on November 1, 2007 at 7:49 AM | PERMALINK

Moving air is less dense than staionary air. That means less drag on the plane, so it moves faster.

This is the reason that the plane is flying in the first place. Because of the curve of the wing, the air flowing over the top of the wing is less dense than the air under it, creating lift.

An airplane flies upside down because the drag it creates by the angle of its wings hitting the oncoming air overcomes the loss of lift from the wing. The Google article explains this. It also notes that planes flying upside down do it for a very short time, just long enough to awe the crowd. A plane flying rightside up and using lift from the wing can fly that way for many, many hours.

Posted by: Lew Wolkoff on November 1, 2007 at 9:18 AM | PERMALINK

Crissa, the weight of the plane makes no difference. Pilots use the same calculation for course correction regardless of the plane they are flying. Similarly, the sweep angle of the wings is not a factor.

Look up "E-3 Calculator". That's the device used. It was invented in the thirties, and is still the tool for the job today.

Posted by: CN on November 1, 2007 at 9:33 AM | PERMALINK

Not to change the subject too much, but you can receive assistance under the school lunch program if your family's income is 185% of the federal poverty level or less. So kids who receive school lunch assistance are not necessarily poor, and if fact most of them aren't. Also, while the percentage of students in private schools nationally has been declining, it's higher in at least some southern states, like Mississippi, where the white kids still go to the private academies set up in response to integration. You know, the ones that Ronald Reagan wanted to grant exemption from taxation. (I very much doubt if they ban black students today.)

Posted by: Alan Vanneman on November 1, 2007 at 10:11 AM | PERMALINK

Everybody who's getting worked up about lift and drag is off on the wrong track. treetop is right, the question is: if the force the plane engine exerts is vector x and the force the wind exerts is vector y, is the magnitude of x+y greater than the magnitude of x? I'm not going to work out the trig in detail, but I believe the answer is yes, so long as the direction of the wind is at 90 degrees, or a little bit behind the direction the plane is traveling. If the force of the wind is directed at all against the force of the engine (say, at 89 degrees), then the answer probably depends on the speed of the wind.

The river problem is the same idea: it doesn't matter which way you swim, the effort to reach either life preserver is the same because you and the life preservers are all moving at the same speed as the current.

Posted by: Chris Conway on November 1, 2007 at 10:45 AM | PERMALINK

Torture can bring a man to talk, but it can't force a man to tell the truth.

Posted by: Jeffrey Davis on November 1, 2007 at 10:49 AM | PERMALINK

I added up the points for the Republicans in the story on health care plans, and McCain came in with 82, one point ahead of Obama! So Kevin, where did they go wrong, because as a good Democrat I know that can't be right.

Posted by: doncoop on November 1, 2007 at 10:49 AM | PERMALINK

so long as the direction of the wind is at 90 degrees, or a little bit behind the direction the plane is traveling.

I'll make this more precise: if 0 degrees is a perfect headwind, 180 degrees is a perfect tailwind, and 90/270 degrees are perfect cross winds, then the magnitude of x+y will be greater than x whenever the angle of the wind is between 90 and 270. If the angle of the wind is between 0 and 89 or 271 and 359, then it is possible for the magnitude of x+y to be greater than the magnitude of x, depending on the magnitude of y (i.e., the force of the wind). (Note here that the plane may be traveling backwards.)

Posted by: Chris Conway on November 1, 2007 at 10:51 AM | PERMALINK

I cannot believe that Alan thinks 'most' families are not poor getting assistance on the school lunch program. A single mother of one child would be hard pressed to provide housing, clothing, food, etc. on less than $2111.00 a month. That is the cut off point in TN.

Posted by: Cautious Clay on November 1, 2007 at 11:52 AM | PERMALINK

National Journal Rating of the Health Plans:

IF you believe they have done a reasonable job in evaluating the various plans one very clear conclusion emerges.

Of the Republican plans McCain's is comes out by far the best. It scored as well as or better than the other two GOP plans on every selected measure except one (incentives for employers to continue coverage 5 vs. 6).

His plan was the only plan to receive no score below 5 on any of the criteria.

Of course, the Democratic plans score significantly higher on many important criteria, but also lower in areas of fiscal impact.

This report suggests a choice between Clinton/Edwards and Obama vs. McCain. Of course, there is a strong argument to be made that to cross the "Chasm" facing healthcare today that none of the approaches of the front runners go far enough.

Posted by: Catch22 on November 1, 2007 at 11:53 AM | PERMALINK

Great link to physics problems in NY Times!
I love the crosswind problem - I am pretty sure I am going to steal it for my Physics I class next semester. I always need new "simple" physics problems like these for class. As someone who teaches physics I find this a great question since if you understand the principles of adding vectors and relative velocity you will be able to answer it correctly without doing anything on a calculator but by application of the principles only.

As some comments have noted this is not a realistic problem (as in real world). A more realistic problem involves adding the vectors but once you understand how to answer the simpler question it is a pretty straightforward step to solving the more complicated (in terms of the math not conceptually) realistic problem.

Posted by: bender on November 1, 2007 at 12:14 PM | PERMALINK

The answer to the crosswind problem is correct even with additional assumptions. The problem says the wind is at 90 degrees to the airplane, not 90 degrees from the destination. Regardless of why you are flying that way, your ground speed will be faster compared to flying with no wind.

The life preserver problem is solved incorrectly, though, for two reasons. One, it says you just fell in the river, so you still have inertia and it will take some time to accelerate. (Someone said that above.) Two, fluid dynamics tells you the water at the top of the river will flow faster than the water towards the bottom, thus the flotation device at the very top will be moving faster than something that is pushed by an average deeper level.

Posted by: Mark on November 1, 2007 at 12:26 PM | PERMALINK

As long as the plane is traveling the direction it wants to go with the side push from the wind, then its ground speed will be higher. But if it actually wants to go the direction it is heading, it will need to adjust course to head slightly into the wind reducing is forward vector...

Posted by: George on November 1, 2007 at 12:29 PM | PERMALINK

I have a PhD. in Physics from UCLA and have taught freshman physics.

Like most over-simplified physics problems which result in lots of discussion, the conundrum is caused by a poorly worded question, not the actual physics of the problem. In a simplified approach all factors like wing shape, drag, lift... are neglected. The problem is supposed to be a simple exercise in velocity frames using Gallilean transformations ( i.e. effects of special relativaty neglected. ) As others have stated before the question should have been stated in terms of a destination (B) that was directly north of the starting point (A) and two separate flights with the absence or presence of a crosswind blowing from directly west.

If two flights are made from A to B with the same air speed, the one with the crosswind present will take longer because the pilot will actually have to fly west at the same speed as the wind to maintain a straight north trajectory with respect to the land.

Just remember, if what is a simple question creates lots of discussion, the problem is with the statement of the problem, not the difficulty level.

However, I do not know whether in this case the teacher screwed up or not. After all this is the NYT reporting, and its probably the fact that the NYT reporter got the story wrong rather than a physics teacher asked a poorly worded problem. But that is just a guess based on how well reporters do at science reporting.

Posted by: John Hansen on November 1, 2007 at 12:30 PM | PERMALINK

Wow, this thread is a Rohrschach test! So much misinformation and so many explanations that are almost correct until the last minute.

Lift is not obtained via drag.
Swept wings have nothing to do with this problem.
This problem is not analogous to a sailboat heading into the wind.
Flight can be explained in purely Newtonian terms.
Bernoulli's principle works quite well in explaining why wings fly, and is part of the explanation of why a plane can fly upside down (so is angle of attack, and so is Newton, and so is engine thrust.)
The plane travels faster for the reason that bob the pilot up above explained.

However, consider this, given all the wrong answers prevalent here that few people realize are wrong, how many wrong things do we incorrectly assume are correct EVERYDAY? And frankly, what does that say about what we think is going on with the dems, republicans, iraq, the media, or even our favorite bloggers?

Remember Sturgeon's Law: 90% of everything is crap. Adjust your filters accordingly.

Posted by: jerry on November 1, 2007 at 12:33 PM | PERMALINK

As others have said, the exact wording of the plane problem would be useful. I doubt that the students were expected to take into account such things as lift and the density of moving air, or did so. I would conjecture that the problem artificially assumes that the wind always stays 90 degrees to the plane's direction, even as the plan turns. If so, I believe that with wind directly from the right, an arc to the right and then back to the left would indeed get the plane there faster than a straight line in calm air.

Posted by: Ken D. on November 1, 2007 at 12:36 PM | PERMALINK

Any student of quantum physics would immediately spot the error. The plane travels both faster and slower than it travels.

Posted by: absent observer on November 1, 2007 at 12:38 PM | PERMALINK

Planes have to crab into the wind when the wind is anything other than 0 or 180 degrees from the direction of intended travel.

You have an east-west lake, 141 mile wide.

You have an aircraft flying 100 MPH.
You have a wind blowing at 100 MPH from the SW to the NE.

Your plane starts 141 miles due south of your destination.

With no wind at all, the plane takes 1.4 hours to cross the lake.

With the wind as described, the plane takes 1 hour to cross the lake, the wind blows 90 degrees to the fuselage, and as bob noted above, the wind actually has a very sizeable tailwind component to the direction of flight.

Now, if I have a 100 mile wide lake, and a plane flying 100 mph, then I cross the lake in one week and ten minutes. I do this by selling enough of my lake and the slow airplane to buy a jet. That takes a week. I hop in the jet and cross the lake in ten minutes.

Posted by: jerry on November 1, 2007 at 12:42 PM | PERMALINK

Imagine the air is still and the ground is moving. Then it is clear you will have to fly farther (since the hypoteneous of a right triangle is longer than the side) to get to your destination so your ground speed is slower.

Posted by: James B. Shearer on November 1, 2007 at 12:42 PM | PERMALINK

I am a physicist and a pilot.

There are too many misconceptions out there (and in the comments above) about both physics and flying, but let me address what seems to be the biggest one.

The "relative wind" on an airplane (i.e. the direction the air is hitting the fuselage) is always forward if the plane is flying correctly, even in a turn. If there's really wind hitting the side of the fuselage, the plane is "slipping," and there's an enormous amount of drag created by that. (Many small planes and gliders slip deliberately to lose altitude quickly, in fact.) If you are slipping constantly, you are not going to be in the air for long.

When there's a 90-degree crosswind to your course, your nose is pointed into the wind just enough to where the plane is meeting the air head-on. With a slower airspeed, it requires a larger angle to account for a given crosswind speed, of course. You may not even notice this "crab angle" on a fast jetliner, but a Piper Cub flying the same course in the same crosswind could be pointed 30 degrees off course.

Also, someone up there said a glider can't change its speed: actually, gliders have very precise control of airspeed.

Posted by: ColoZ on November 1, 2007 at 12:52 PM | PERMALINK

Dear Jerry,

90% of your posts are crap. Hope that helps!!!!!!


Posted by: Treetop on November 1, 2007 at 12:57 PM | PERMALINK

Ah! But 10% of my posts are cream!

Posted by: jerry on November 1, 2007 at 1:00 PM | PERMALINK

So, as the comments above show, the plane does fly faster. The additional speed is at right angles to the nominal direction of the plane, though.

Anonymous said something wrong above: "The equivalent of a boat keel is that big airplane tail." That's flat wrong. The keel keeps a sailboat from traveling perpendicular to it's nominal direction because the keel is in the water. (Actually, there's still some leeway, and the amount depends on the boat, the ocean conditions, the wind, ...)

The plane is in the air, so there's nothing else which can be used to compensate for the wind.

Posted by: Amit Joshi on November 1, 2007 at 1:02 PM | PERMALINK

Looking back, the sex offender story deserves its own post. The prison lobby is one of California's largest and most powerful lobbies. I assume that's true all over and in Wisconsin.

We do many things that are expensive and have no benefit. Locking sex offenders down on Halloween is one of them. We do this and we build up expensive infrastructures that are hard to knock down. We do this and build up additional lobbying forces that are hard to knock down.

What percentage of sex offenders these days are for things like statuory, but consensual sex? Or for getting pics of your kids developed at the walmart, or sending pics of yourself or your boyfriend across the net?

It's a non-solution to a problem that doesn't exist.

Posted by: jerry on November 1, 2007 at 1:11 PM | PERMALINK

I don't see the "punch line at the end" of the sex offender story. And as you have no children of your own, Kevin, you're going to view the issue differently than parents. I don't believe sex offenders should ever be paroled. It's a form of mental illness that has resisted therapy to date.

Posted by: JeffII on November 1, 2007 at 1:33 PM | PERMALINK

Health care scores:
McCain 77
Guliani 60
Romney 59

So Guliani and Romney are pathetic, but McCain's did well enough that there might well be some ideas in it that are worth incorporating into the Clinton/Obama/Edwards plans.

Posted by: Tom Veil on November 1, 2007 at 2:13 PM | PERMALINK

As a Naval Architect and sailboat designer Amit Joshi is right that the tail is nothing like a keel, and the cross wind thing has little to do with a sailboat, unless you are talking about a sailboat in waters with current.

As far as the physics question goes, the question doesn't say the wind is 90 degrees from the destination, just that it is 90 degrees from the plane's direction, and therefore, the groundspeed is increased. I believe most pilots would consider this to be a slight tailwind as they would generally refer to the direction they needed to go, not the direction they point the plane.

In sailing, this is common, but in a slightly different manner. We almost always have a leeway angle when sailing upwind. Unless you have asymetric daggerboards, or a trim tab to make your keel asymetric you need leeway for the keel to make lift. Your compass will always read slightly different than your course over ground even if perfectly calibrated because the boat is not pointed in the direction of travel.

Posted by: Gram on November 1, 2007 at 2:33 PM | PERMALINK

An African swallow or a European swallow?

Posted by: trollhattan on November 1, 2007 at 2:53 PM | PERMALINK


But, your comments on the sex offender post don't address the "family abuser" issue. Families fight prosecution on issues like this, the more so the stiffer the penalty.

Posted by: SocraticGadfly on November 1, 2007 at 3:20 PM | PERMALINK

In the river, assuming there is no wind (or not knowing the speed and direction of the wind) you should swim toward the downstream object. It will be moving slightly slower than the flow due to aerodynamic drag from the part of it sticking up above the water. You are less buoyant than the preserver (hopefully) so less of you is sticking up into the air and so you will be less effected by the aerodynamic drag than it is.

Posted by: jefff on November 1, 2007 at 3:31 PM | PERMALINK

You swim to the preserver behind you because it's easier to catch one coming towards you that you can trap with your body and arms than one traveling away from you that you have to reach for. You swim to the preserver behind you because the dam is in front of you.

Posted by: jerry on November 1, 2007 at 3:41 PM | PERMALINK

I have a Ph.D. in Nuclear Physics (really), but I am only pretty good with Rocket Science. Still, imagine a 300 mph plane in a 500 mph wind. If you fly with wind you will go really fast, but if your intended destination is anything but downwind, you will never get there.

Posted by: fafner1 on November 1, 2007 at 3:41 PM | PERMALINK

"You swim to the preserver behind you because it's easier to catch one coming towards you that you can trap with your body and arms than one traveling away from you that you have to reach for. You swim to the preserver behind you because the dam is in front of you."

The preserver isn't coming toward you. Without swimming, and with no wind, it actually ought to be being slowly dragged away from you by air resistance.

If there is a dam, waterfall, etc depending on the time till you go over/through it you might swim forward or back. You might also just swim for shore if the life preserver isn't going to help.

Posted by: jefff on November 1, 2007 at 3:44 PM | PERMALINK

The plane/crosswind problem is not bogus, but getting a practical answer depends on unstated coincidental conditions. If you start out flying directly toward your destination and the wind is perpendicular to the start-finish vector, your ground speed is higher than the air speed, but you don't arrive.

The answer would be practical if the wind were not exactly 90 degrees to the start-finish vector, but blows with somewhat of a tailwind component. Then you would head somewhat to the side of the destination, and the ground speed would be resultant of your airspeed and the sidewind speed (as it always is). This is a general solution to the problem of taking the optimum air heading. It would be essentially coincidental if the sidewind turns out to be exactly 90 degrees to your heading.

Posted by: skeptonomist on November 1, 2007 at 4:56 PM | PERMALINK

The "punch line at the end", jeffII, are these quotes.

"There has not been a single case of any child being molested by a convicted sex offender while trick-or-treating," writes columnist Benjamin Radford on LiveScience.com.

"Here we're creating a new police action squad to go out and address a problem that has never manifested itself in the community," Gresback told the newspaper. He said in 20 years he'd never run across a case of a sex offender attacking a child on Halloween.

JeffII, Kevin might not have children, but I do. I want my children to be safe from child molesters. I also want my entire family to be safe from terrorists. For neither cause do I advocate panicked and unnecessary reactions, because (among other reasons) they just make the problem worse.

(One of my problems with treating all "registered sex offenders" as a single group are that in many states -- like mine -- many of these "registered offenders" are men who engaged in sodomy with other consenting men, back when it was a sex crime to do so.)

Posted by: eyelessgame on November 1, 2007 at 5:01 PM | PERMALINK

I think too many people here are over-analyzing the crosswind problem. The question clearly asks for the speed relative to the ground not the time to get to a destination.

Posted by: bender on November 1, 2007 at 5:11 PM | PERMALINK

[Attention physics nerds: the New York Times says a plane flying through a crosswind travels faster than it would in clear skies.]

Oh Good Grief.

You Humans failed your IQ test. It's a trick question, but funny as heck to read the posts.

Read the sentence...again:

"a plane flying through a CROSSWIND travels faster than it would in CLEAR skies."

Clear does not mean ZERO wind speed.

What the heck does a CROSSWIND got to do with CLEAR skies!!!

P.S. When does my starship come and pick me up?

Posted by: James on November 1, 2007 at 6:34 PM | PERMALINK

P.S.S. The real question was:

A plane flying into a headwind will have a lower speed, relative to the ground, than it would if it were flying through still air, while a plane traveling with the benefit of a brisk tailwind will have a comparatively greater ground speed. But what about a plane flying through a 90-degree crosswind, a breeze that is buffeting its body side-on? Will its ground speed be higher, lower or no different than it would be in unruffled skies?

Kevin wrote a misleading question; either on purpose or unintentional.

Either way it was fun to watch the posts.

Correct question answer: (Assuming no other forces on the plane but the crosswind and the plane does not change it's flying direction): The plane's speed going forward will not change.

However, the plane will be blown off course as the forward speed couples with the crosswind to create a resultant speed at an angle to the planes forward motion.

Destintion ----->x X / arrive.
forward motion ^ *
| /
| / | / plane gets blown this way-->
crosswind-----> A

Posted by: James on November 1, 2007 at 7:03 PM | PERMALINK

Another interesting question is: How would a stiff, but directly from the side wind affect the top speed of an automobile (and why)?

Posted by: jefff on November 1, 2007 at 7:29 PM | PERMALINK

Anyone else notice MISSING comments on this thread?

I posted links to TWO articles last night, neither of which is here today.

VERY cryptic.I've never seen comments simply DISAPPEAR (without any trace) at WM/PA before.

Posted by: Poilu on November 2, 2007 at 5:03 AM | PERMALINK

Note: The above (deleted?) comments were posted November 1 roughly between 4:00 and 6:00 AM.

I DO notice a gap for that interval above. Gremlins??

[When posts containing links disappear that usually means they have been snagged by Moveable Type's anti-spam engine by mistake. Given the time you posted them there were no moderators around to publish them. Apologies for the inconvenience -- mod.]

Posted by: Poilu on November 2, 2007 at 5:24 AM | PERMALINK

Poilu, for whatever reason, they've been moderating comments here for sometime, and yes, that means that comments can be silently tossed down the memory hole.

Apart from the Chinese Spam that would show up from time to time, I do not think that moderating comments adds any real value. I don't know what caused yours to hit some moderators trigger, and they rarely tell us.

Somewhere here is an email address that you can use to discuss it.

Posted by: jerry on November 2, 2007 at 4:49 PM | PERMALINK

Thanks for the feedback, Mod and Jerry.

Being as I had verified the two postings after the fact (i.e., before they vanished), I initially assumed spam filters were not the cause, since they usually prevent a comment from actually posting without subsequent approval.

But, whatever. Much obliged.

Posted by: Poilu on November 4, 2007 at 5:22 AM | PERMALINK



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