# Crappy science instruction **URL:** \ **Category:** The BBQ Pit\ **Created:** [October 25, 2012, 3:24pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830 "2012-10-25T15:24:51Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Johanna](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johanna/32/8318_2.png) [@Johanna](https://boards.straightdope.com/u/Johanna)\ **Post date:** [October 26, 2012, 4:25am UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/21 "2012-10-26T04:25:12Z")
Let us hie to the stone-extraction site and propel projectiles earthward therein. ---
**Author:** ![Autolycus](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Autolycus](https://boards.straightdope.com/u/Autolycus)\ **Post date:** [October 26, 2012, 4:52am UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/22 "2012-10-26T04:52:29Z")
Yes, ok, so as men of science we should freely admit that the first experiment demonstrates convective cooling instead of adiabatic pressure-induced thermodynamic flux, but, what if, I say what if the experimenter pursed his lips and blew air not directly into the air but through the most venerable of all scientific instruments, a paper towel tube? Aha! Hewitt doesn’t look so wrong now, does he? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [October 26, 2012, 5:26am UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/23 "2012-10-26T05:26:37Z")
> [@Autolycus](#): > > Yes, ok, so as men of science we should freely admit that the first experiment demonstrates convective cooling instead of adiabatic pressure-induced thermodynamic flux, but, what if, I say what if the experimenter pursed his lips and blew air not directly into the air but through the most venerable of all scientific instruments, a paper towel tube? Aha! Hewitt doesn’t look so wrong now, does he? Pervert ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [October 26, 2012, 5:29am UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/24 "2012-10-26T05:29:30Z")
A Youtube video is wrong about science? My world lies in tatters. ☹ ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [October 26, 2012, 7:54am UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/25 "2012-10-26T07:54:10Z")
> [@Johanna](#): > > Let us hie to the stone-extraction site and propel projectiles earthward therein. You’re just _begging_ to be thrown into a quarry, here. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [October 26, 2012, 8:06am UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/26 "2012-10-26T08:06:20Z")
> [@Chronos](#): > > Absolutely everything in the Universe, from subatomic particles to supermassive black holes, obeys Newton’s Third Law, with no exceptions. And it is in fact one of a number of principles that is quite relevant to how a rocket works. I broke Newton’s Third Law once. The Third Law responded by breaking me. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [October 26, 2012, 12:05pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/27 "2012-10-26T12:05:07Z")
> [@njtt](#): > > The real question is, can you **Machine Elf** , come up with a better way of explaining adiabatic expansion to kids, that will catch their imaginations at least as well (that bit is important), but will remain as strictly accurate as you would wish? If you can, fine. It will, I suppose, be marginally better in educational terms. If you can’t then you need to shut up and let educators get on with effective educating, instead of trying to hobble them with pettifogging pedantry. I wouldn’t have minded so much if the _majority_ of the cooling effect he observed were due to adiabatic expansion. That would have been a matter of him being “mostly correct,” as was the case for Newton and Aristotle. But in the video, he was mostly wrong, and I think that’s a disservice to his students. Explaining the effect verbally isn’t too hard. Coming up with practical examples that all or most of the students can relate to? That’s slightly more difficult, given that most folks don’t often encounter situations in their daily life where they directly experience the effects of adiabatic expansion/compression. But not impossible. One that I can think of involves using a hand pump to inflate a bicycle tire: by the time you’re done, the tire’s valve stem will be hot to the touch, because you’ve been compressing air to 60 psi (or 100 psi, depending on the tire), which heats it up before jamming it through the valve stem and into the tire. Conversely, any mechanic who has changed a car or motorcycle tire knows that when you remove the valve core to let all the air out, the valve stem will be cool to the touch (_after_ the air has stopped flowing), because of adiabatic expansion of the air as it exits (the effect is there on bicycle tires too, it’s just harder to observe because the tire is typically emptied before the valve stem gets very cool). Those are two demos you easily could do in the classroom. Start with a half-inflated bicycle tire (to save time), and have a volunteer pump it up to full pressure, then feel the valve stem. Likewise, start with a fully inflated car tire (for best effect start with a 60-psi mini-spare), remove the valve core and let it deflate - and then have a volunteer feel the valve stem. Another really good one for the classroom is a [fire piston,](http://www.youtube.com/watch?v=Q3RX64DbciM&feature=relmfu) which demonstrates that if you compress air enough, you can get it hot enough to actually ignite things. ---
**Author:** ![Happy\_Fun\_Ball](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/happy_fun_ball/32/17664_2.png) [@Happy\_Fun\_Ball](https://boards.straightdope.com/u/Happy_Fun_Ball)\ **Post date:** [October 26, 2012, 2:16pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/28 "2012-10-26T14:16:53Z")
> [@Hyperelastic](#): > > Google “How a rocket works”. You’ll likely get a pile of links that say it’s because of Newton’s Third Law. Even NASA sites claim this. > > In reality Newton’s Third Law has little to do with how a rocket works. In fact, there are ion thrusters whose exhaust doesn’t even obey Newton’s Third Law and they work just fine. > > The operation of a rocket is explained by Newton’s Second Law generalized to open systems. What are you talking about? ---
**Author:** ![Virgil\_Tibbs](https://avatars.discourse-cdn.com/v4/letter/v/c4cdca/32.png) [@Virgil\_Tibbs](https://boards.straightdope.com/u/Virgil_Tibbs)\ **Post date:** [October 26, 2012, 3:17pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/29 "2012-10-26T15:17:59Z")
How about giving students CO2 fire extinguishers and allowing them to give each other frostbite? Or would schools not allow it? ---
**Author:** ![Darth\_Panda](https://avatars.discourse-cdn.com/v4/letter/d/ee7513/32.png) [@Darth\_Panda](https://boards.straightdope.com/u/Darth_Panda)\ **Post date:** [October 26, 2012, 4:01pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/30 "2012-10-26T16:01:27Z")
In this house, we obey the laws of motion! ---
**Author:** ![Hyperelastic](https://avatars.discourse-cdn.com/v4/letter/h/a3d4f5/32.png) [@Hyperelastic](https://boards.straightdope.com/u/Hyperelastic)\ **Post date:** [October 26, 2012, 4:50pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/31 "2012-10-26T16:50:59Z")
> [@Chronos](#): > > Absolutely everything in the Universe, from subatomic particles to supermassive black holes, obeys Newton’s Third Law, with no exceptions. And it is in fact one of a number of principles that is quite relevant to how a rocket works. Newton’s Third Law says that particles exert equal and opposite forces on each other. In electrodynamics, particles do not exert equal and opposite forces on each other. Look at page 148 of _Principles of Electrodynamics_ by Schwartz. ---
**Author:** ![lieu](https://avatars.discourse-cdn.com/v4/letter/l/c2a13f/32.png) [@lieu](https://boards.straightdope.com/u/lieu)\ **Post date:** [October 26, 2012, 4:59pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/32 "2012-10-26T16:59:29Z")
> [@Machine\_Elf](#): > > My wife found a YouTube video last night that purports to demostrate the cooling associated with adiabatic expansion of gases. Mine too, only it was a typo. ---
**Author:** ![Furious\_Marmot](https://avatars.discourse-cdn.com/v4/letter/f/dfb087/32.png) [@Furious\_Marmot](https://boards.straightdope.com/u/Furious_Marmot)\ **Post date:** [October 26, 2012, 6:54pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/33 "2012-10-26T18:54:49Z")
> [@Virgil\_Tibbs](#): > > How about giving students CO2 fire extinguishers and allowing them to give each other frostbite? Or would schools not allow it? Our 7th grade science teacher used to blast a CO2 extinguisher into the air for exactly this purpose. Snow, noise, adiabatic expansion, perfect middle-school lesson. One year, she was unaware that the extinguisher had been replaced with the dry chemical variety. Doused everybody with fine yellow powder, and pissed off the janitor. ---
**Author:** ![ads95](https://avatars.discourse-cdn.com/v4/letter/a/5daacb/32.png) [@ads95](https://boards.straightdope.com/u/ads95)\ **Post date:** [October 26, 2012, 7:38pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/34 "2012-10-26T19:38:34Z")
> [@Virgil\_Tibbs](#): > > How about giving students CO2 fire extinguishers and allowing them to give each other frostbite? Or would schools not allow it? You could get a can of “Dust-Off”, which is really just a can full of a fluorocarbon blowing agent (1,1 difluoroethane). Hold it upright and squeeze the trigger, you just expand vapor across the valve. It is a tiny bit colder than the can, but not really discernable. Then hold the can upside down and squeeze the trigger. You will be flashing liquid in the can across the valve, and the resulting vapor leaving the can is much colder. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 26, 2012, 7:39pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/35 "2012-10-26T19:39:06Z")
> [@](#): > > In electrodynamics, particles do not exert equal and opposite forces on each other. Look at page 148 of Principles of Electrodynamics by Schwartz. A more accurate statement of the law would be that _things_ exert equal and opposite forces on each other. I’m actually rather fond of the example that you mention, because it illustrates that the particles are _not_ actually exerting forces on each other, and that there’s another thing in the problem that is exerting forces and having forces exerted on it. But the force of particle A on the field is, in fact, exactly equal in magnitude and opposite in direction to the force of the field on particle A, and likewise for B. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [October 26, 2012, 8:24pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/36 "2012-10-26T20:24:04Z")
> [@ads95](#): > > Then hold the can upside down and squeeze the trigger. You will be flashing liquid in the can across the valve, and the resulting vapor leaving the can is much colder. Also crappy science instruction. If you’re flashing liquid to vapor, now you’re demonstrating evaporative cooling, not adiabatic expansion. ---
**Author:** ![Hyperelastic](https://avatars.discourse-cdn.com/v4/letter/h/a3d4f5/32.png) [@Hyperelastic](https://boards.straightdope.com/u/Hyperelastic)\ **Post date:** [October 26, 2012, 10:14pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/37 "2012-10-26T22:14:47Z")
> [@Chronos](#): > > A more accurate statement of the law would be that _things_ exert equal and opposite forces on each other. I’m actually rather fond of the example that you mention, because it illustrates that the particles are _not_ actually exerting forces on each other, and that there’s another thing in the problem that is exerting forces and having forces exerted on it. But the force of particle A on the field is, in fact, exactly equal in magnitude and opposite in direction to the force of the field on particle A, and likewise for B. That interpretation came long after Newton’s day. Rather than argue the fine point of whether a particle can exert a force on a field, let me amend my statement to: According to standard textbooks such as those of Schwartz and Goldstein, the Biot-Savart force violates Newton’s Third Law. Regardless, the equation of motion of a rocket has nothing directly to do with Newton’s Third Law. The rocket exerts a force on its exhaust and the exhaust exerts an equal and opposite force on the rocket - so what? That’s true when the rocket is still bolted to the launch pad, and for that matter is true of the force a building exerts on its foundation. The usefulness of a rocket is that it accelerates, but Newton’s Third Law says nothing about motion. Explaining the operation of a rocket solely in terms of the 3rd Law, as [this NASA site](http://www.nasa.gov/audience/foreducators/rocketry/home/what-is-a-rocket-k4.html) does, is no explanation at all. On the other hand, Euler’s generalization of Newton’s Second Law to an open system gives the rocket’s acceleration in terms of the forces on it and the rate of change of its mass. ---
**Author:** ![Weedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/weedy/32/301_2.png) [@Weedy](https://boards.straightdope.com/u/Weedy)\ **Post date:** [October 28, 2012, 9:16am UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/38 "2012-10-28T09:16:36Z")
> [@Hyperelastic](#): > > On the other hand, Euler’s generalization of Newton’s Second Law to an open system gives the rocket’s acceleration in terms of the forces on it and the rate of change of its mass. This is maybe more of a General Question, but what are the forces acting on the rocket that make it move? ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [October 28, 2012, 7:53pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/39 "2012-10-28T19:53:45Z")
> [@ads95](#): > > You could get a can of “Dust-Off”, which is really just a can full of a fluorocarbon blowing agent (1,1 difluoroethane). > > Hold it upright and squeeze the trigger, you just expand vapor across the valve. It is a tiny bit colder than the can, but not really discernable. > > Then hold the can upside down and squeeze the trigger. You will be flashing liquid in the can across the valve, and the resulting vapor leaving the can is much colder. Yick. Don’t do that now, though; they started putting a bitterant in Dust-Off (and every other brand I’ve ever seen) a few years ago to discourage inhalant abuse. Now, you don’t even want to spray too much gas at once, or things start to taste really, really bad just from just from the gases in the air. And definitely don’t touch anything that has gotten the liquid on it until it’s been washed thoroughly. And don’t eat anything after using it unless you’ve washed your hands off thoroughly too, because there’s a good chance your food will taste horribly bitter. ---
**Author:** ![Baron\_Greenback](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/baron_greenback/32/48_2.png) [@Baron\_Greenback](https://boards.straightdope.com/u/Baron_Greenback)\ **Post date:** [October 28, 2012, 8:47pm UTC](https://boards.straightdope.com/t/crappy-science-instruction/638830/40 "2012-10-28T20:47:54Z")
> [@Machine\_Elf](#): > > My wife found a YouTube video last night that purports to demostrate the cooling associated with adiabatic expansion of gases. The problem is that it doesn’t actually demonstrate what the instructor in the video claims it demonstrates. Was it one of the Cake Fart videos? There’s certainly gaseous expansion going on, not sure if it adiabatic though. [Previous page](https://boards.straightdope.com/t/crappy-science-instruction/638830.md?page=1) [Next page](https://boards.straightdope.com/t/crappy-science-instruction/638830.md?page=3)