Ask MetaFilter questions tagged with physics

Most effective means of moving hot air?

ascetic — Wed, 16 Dec 2009 13:24:48 -0800

What is the best way to move hot air? Would it be blowing air across the heat source (ex: wood stove) or suck the air away from the heat source? I use a wood stove to heat my home which works perfectly, but it occasionally gets too hot in the living room (where the wood stove is). I want to disperse some of this excess heat into the other rooms.

I've just spent 16 hours on a lab. Never again.

vogon_poet — Sun, 13 Dec 2009 18:30:33 -0800

What spreadsheet/graphing software should I use to do physics labs? I am a high-school student, enrolled in the International Baccalaureate program. I'm taking an IB Physics class, and there's a lab component that's pretty substantial. What's worse, my physics teacher has his own very high expectations for labs, and trying to fulfill the standards of both is pretty difficult. Typically the labs are between 50-100 pages, about 60% graphs or charts. There is always too much data to do anything manually and maintain my sanity.

IB has a lot of bizarre niggling requirements for these labs. Every number has to have a unit and uncertainty associated with it (I'm not allowed to just put "+/- .5 V" at the top of a column) and significant figures are vitally important. Indeed, even a single error on one datum will drop me by the IB equivalent of a full letter grade, at least according to my teacher.

So, I would like some spreadsheet/graphing software that can do what I need automatically.

As far as numbers and calculations go:
-- there needs to be a way to associate units and uncertainties to numbers and still use them in calculations
-- calculate while propagating uncertainties
-- perform calculations using the proper numbers of significant figures
-- easily export charts, with units and uncertainties printed next to each number, for insertion into a word-processing document

As far as graphing goes:
-- It needs to be able to handle error-bars on graphs, and perform at least linear regression.

Does anything like this exist? I've looked around for formulas to make Excel work the way I want it to, but most of them are unsatisfactory. I've also tried out R, but I don't have time to program all this myself, and R seems pretty difficult to learn anyway.

Plan B is amphetamines

walking through walls

bbuda — Sat, 05 Dec 2009 18:56:43 -0800

Why don't I fall through chairs? Physics geeks needed. A friend and I, who both have dangerously little knowledge of physics, are trying to determine why solid objects can't pass through each other (given that atoms are mostlÝ 'empty space'). He says that the answer involves virtual photons and the Pauli Exclusion Principle. I say there must be a simpler, classical reason (Occam's razor and all). Which one of us is less wrong?

Tips on understanding concepts on a deeper level?

amillionbillion — Fri, 04 Dec 2009 08:45:47 -0800

How can I better approach learning intro physics? I'm a post-bac premed and college-level (non-calculus) physics is kicking my ass. I know that physics isn't about mechanically plugging numbers into formulas, but how do I achieve a deeper conceptual understanding? I read the textbook thoroughly, chug through the homework, and I'll think I get the topic, but if faced with a problem that's twisted a bit differently (e.g. killer exam questions), I'm at a loss. I have no idea how to apply what I know to unfamiliar problems, which means I didn't really get it at all. I rarely have that "a-ha" moment of clarity. My professor is horrible, and tutors seem to focus on problem-solving instead of teaching me how to *think* about physics. (And this is only mechanics/kinematics right now; I predict if the trend continues, I'll be totally screwed in electromagnetism.)

The final is 2 weeks away and 45% of the final grade, so I still have one last chance...

I know what I'm looking for sounds a bit vague, but I'd appreciate any advice on how to learn something that's abstract, and thinking about concepts from different angles, as well as how to stay motivated and even psyched about learning. I'm kind of down on physics right now, and filled with this sense of dread/fervent wish I were Isaac Newton, which doesn't help.

Twinkle Twinkle little present?

jennyhead — Fri, 27 Nov 2009 07:14:30 -0800

Burgeoning Astronomer-filter: My husband is getting interested in the cosmos and theoretical physics. Any neat gift ideas to encourage this? I've googled the interweb but don't know where to start.

Seeking clicking mechanism or design...

pallen123 — Sat, 21 Nov 2009 08:21:02 -0800

What kind of small toys or devices click when you rotate them forwards and backwards? Seeking mechanism or design... I’m trying to help a friend design a small toy and she needs a mechanism that will click in very small increments as she rotates a small (1 inch diameter) dial that turns 360 degrees on its axis. She needs the dial to click precisely as it turns clockwise and counterclockwise in smallish increments. I was thinking a fishing reel mechanism might work, but that just allows turning in one direction. Then I thought a ratchet wrench mechanism might work, but that also turns in just one direction (and I’m not sure how the inside of a ratchet wrench works either). Can anyone think of existing toys or devices that have a clicking mechanism that freely rotate? Novel solutions? Suggestions? Thank you!

What's a good idea for a thesis paper combining English and Physics?

Archibald Edmund Binns — Wed, 18 Nov 2009 18:25:49 -0800

I go to a small Liberal Arts school in Pennsylvania, and part of the graduation requirements involve writing a substantial Senior Composition, basically a baby thesis (Original research requirements, rigorous, etc.) I'm double-majoring in Physics and English, and I'd love to be able to combine both of these subjects into one comprehensive paper, especially because if i can't, I have to write two separate ones. I've tossed a few ideas around (Analysis of Quantum Mechanics in Orson Scott Card's Xenocide/Children of the Mind; A series of several parables designed to illustrate some facet of quantum mechanics to the average person, etc.), but all of them are mostly English-based and only superficially deal with Physics. (The Physics professors tend to be the harder ones to convince, most English professors are happy to do something different). Any ideas?

Can my chronomancer dodge bullets?

Caduceus — Wed, 11 Nov 2009 13:11:46 -0800

I have some weird, tricky physics questions based on a fantastic (non-real) premise, involving bullet speed and potential injury. I'm toying with the writing of a fantasy-action story/novel set on the battlefields of the first World War. One of the characters is a wizard-type with the ability to control his own personal flow of time. The simplest explanation is that he can make it so that for every one second that passes for the rest of the world, ten seconds pass for him. I'm not sure how this would work if someone was able to do it in reality, but for purposes of the story, it means both that he moves ten times as fast as everyone else, in a fast-forward sort of way, and that anything that might be able to affect him moves 1/10th the speed.

My questions involve the effect of bullets shot at him. He's on the side of the Allies, so as his major concerns are German bullets, I've been referencing this, this, and this Wikipedia page, but I just plain don't know how to do the physics anymore.

Based on my premises, it seems that the effective muzzle velocity for a German rifleman shooting at him should be 87.8 m/s, instead of 878 m/s as the Gewehr 98 page places the real velocity. My first question is, would a bullet going that fast be visible or dodgeable?

Second, the ammunition page says that the weight of the 1905 version of the round (which is the one they used in the war) is 9.9 grams. If I'm doing my math right, that means the momentum of the bullet should be 0.86922 kg*m/s. However, I have no idea how to turn that into force, or what in general it means in terms of it actually hitting the protagonist. If he were hit in his time-shifted state, how much would it hurt him?

Third two-part question: if he could not dodge bullets at 1/10th speed, how much would he have to "slow time" to be able to dodge them. If 1/10th speed bullets would still be lethal, at what point would they not be lethal?

Finally, as a bonus question, what could he do to his enemies? If he were to throw a punch while time-shifted, what effect would that have on the punchee, given that he's effectively throwing his punch in 1/10th of the time as a normal punch is thrown? Would that be ten times the force? One hundred times the force? What would that do to someone in real world terms?

I would be happy to just be given a decent idea of the answers I'm looking for, but if anyone can show me how to do the physics despite the strange starting premise, I'd be really thrilled. Thanks!

What's more important, a magnet's surface area or thickness?

howling fantods — Thu, 05 Nov 2009 11:27:53 -0800

What contributes more to the attractive force between a magnet and a thin piece of metal - the magnet's surface area, or its thickness? Suppose I have thin piece of metal and a neodymium disc magnet. With everything else being equal, will a thin magnet with a large surface area provide a stronger pulling force than a thicker magnet with a smaller diameter? In either case, the piece of metal is no thicker than the magnet itself.

Can anyone suggest interesting careers for a physicist hoping to make a good impact on society?

qmechanic — Thu, 29 Oct 2009 22:30:05 -0800

Can anyone suggest interesting careers for a physicist hoping to make a good impact on society? Currently, I'm a theoretical physics graduate student at an Ivy League school and for various reasons I don't want to discuss, I'm quitting and getting out with the MA. The fact is that I already have two bachelor's degrees in physics and EE as well as a master of engineering from the other university in Cambridge, MA (not Harvard), so I don't feel like the PhD would add too much.

I've been researching career paths and clean energy is an obvious choice. I'm good at modelling and I can see that being a useful skill in the clean energy industry. Another option would be health care. I have a friend does modelling at a health care company. I like clean energy better than health care since it's a newer field with less bureaucracy.

Besides these two, I'm having trouble coming up with other ideas. My criteria are basically this: 1) I want to use my scientific skills and 2) I want to work on projects with broad social impact. Intellectually challenging work would be a plus. I'm willing to do some more school, though probably not a whole another PhD.

I welcome any suggestions, as long as they fit the two criteria I mentioned.

How can I calculate side force on my car?

krisak — Thu, 29 Oct 2009 20:22:57 -0800

Physics question - involving cars, air, momentum, force, etc... Basically - how can I calculate the side force from a crosswind on my car at any given speed? (Yes, I googled as much as I could and couldn't come up with a formula for this.)

First off, I drive a 2001 Mazda Miata. 155.7 inches long by 48.4 inches tall. Considering the shape of the car, let's assume 155x35 inches as a side profile.

I'm not sure if the front dimensions are important, but I can measure the front end if necessary. For the sake of the math, I think we should disregard the front end of the car - I don't know what the drag ratio and force necessary to push through the air are on this car.

I guess my real question is how do I come up with a figure for how much force a cross wind applies to my car? And, at any given speed, how does that translate into lateral acceleration? (I understand I may be asking the wrong question here.) I know that a 10 mph crosswind will have a different effect when the car is traveling at 5 mph versus 50 mph.

This question comes out of the fact that while driving to work this morning I almost couldn't change lanes due to an 85 mph forward speed coupled with a 50 mph cross wind. I know that at a 10 mph forward speed the effect of that side wind would be lessened, I just want to understand the math behind why.

I apologize in advance for being unable to phrase this question properly. :) Perhaps part of this question is meant to ask "how do I phrase this properly such that I can get a solution to the question?"

Bicycles & physics

bluelight — Thu, 29 Oct 2009 06:49:13 -0800

bicyclefilter: Are there any sites or books that break down the science of a bicycle? I want to understand how a bicycle works. I have a pretty good bicycle mechanics book (Zinn and the art of mountain bike maintenance) but I want to gain a deeper understanding.

I don't have much of a physics background, but I want to understand things such as how/why the rear derailer pulley system works and how mechanical advantage and the braking system are related.

The Sheldon Brown website covers this a little bit (he does discuss MA) but I'm still looking for more sources.

Thanks!

The twenty most important numbers

bigmusic — Fri, 23 Oct 2009 14:13:51 -0800

In a talk (at TED) by Brian Greene on string theory he says that there are "there appear to be about 20 numbers that really describe our universe..." He lists a few in his talk, but what are the rest of of those numbers? The pertinent excerpt from the talk:



And that question is this: when we look around the world, as scientists have done for the last hundred years, there appear to be about 20 numbers that really describe our universe. These are numbers like the mass of the particles, like electrons and quarks, the strength of gravity, the strength of the electromagnetic force -- a list of about 20 numbers that have been measured with incredible precision, but nobody has an explanation for why the numbers have the particular values that they do.

Uh...hang on, just give me a minute.

Demogorgon — Mon, 12 Oct 2009 10:34:31 -0800

What is the most important scientific question of our time? I volunteer at an observatory for a local amateur astronomers' society and one of the guests at a recent star party came up and asked, "What do you think is the most important question science has to answer right now?" Obviously, there is no right or wrong answer, but after the party was over a lot of us were still talking about this question and I ended up learning a great deal from my fellow club members that I might otherwise not have.
The next time this question gets asked I want to be prepared to offer a variety of answers from differing fields and opinions. I don't expect to represent every answer as an expert, but I'd like to be able to give a few more examples than I was able to, and then correlate them to some book recommendations from the answers in this thread about introductions to your field.
I also think it is important to frame the question in a way that can be meaningfully answered, i.e. "What is the most important scientific discovery about to be made?" or something like that.
Of course, I had my own answer in mind, but as a relative layperson to that branch of study I had a really hard time articulating why it was so important to "science." Therefore, if you are uniquely affiliated with a specific field that you think will produce a game-changer, feel free to get as technical as you're comfortable doing.

Thanks!

Quotes for a science classroom wall

alby — Mon, 28 Sep 2009 10:11:09 -0800

I'm looking for good quotations about science (particularly physics) and education that are suitable for the wall of a secondary school (ages 11-18) classroom. As examples, these are quotes I've already used:
"The two most common elements in the Universe are hydrogen and stupidity." - Harlan Ellison
"Physics is like sex. Sure it may give some practical results, but that's not why we do it" - Richard Feynman
"The illiterate of the 21st Century will not be those who cannot read and write, but those who cannot learn, unlearn, and relearn." - Alvin Toffler

I don't understand physics. How do atomic orbitals work?

jeb — Mon, 21 Sep 2009 19:39:32 -0800

I don't understand physics. How do atomic orbitals work? I don't know anything about modern physics beyond the "I watched a trippy special on the Discovery Channel" level Consider this situation: I have one atom of hydrogen. Bound to it is a single electron in the 1s orbital. This electron has a negative charge. As I understand it, the orbital describes a probability density function: if I sample a point in the physical space described by the orbital, the value of that point in associated probability density function is the probability that I will encounter the electron there when I look for it (bonus side question: so if I integrate across the three dimensions of the orbital do I end up with 1?).

Let's assume I set up the following apparatus: I have my atom. At a fixed point from my atom, I have a very tiny electric field meter. This meter will record the strength of electric fields you put it in. I start recording the field strength. What do I see?

a) an oscillating value: the electron being a charged particle creates an electric field that falls off with distance to the meter. When the electron is closest to the meter, it registers the strongest field, and when it is further, it registers the weakest. The rest of the time it careens through points in between (bonus side question if this is the answer: is the output of the meter smooth, continuous, differentiable, etc., because the electron is careening through the physical space the orbital is defined over, or is it just bliz-blaz-bloz because the electron is teleporting to wherever it feels like?)

b) a fixed value: the field the meter would pick up if a point charge equal to the electron was fixed at some point (i don't know, maybe the closest point?) to the meter.

c) a different fixed value: something like the average charge. because quantum mechanics is mad trippy, the charge meter picks up, at all times, the sum of the charge it would detect from a point charge at each point in the orbital over the volume of the orbital.

d) something completely different.

e) Moo. The poster's understanding of the atom is so far wrong that this question cannot be answered.

(Super-bonus question: I'm not in school or anything, but I still want to learn some basics about how these things work. Is this physics or chemistry? Personally, I'm more interested in going up from here into how these work in groups than down into what makes up the various components at this point. Can you recommend any resources whereby I can teach myself? On Amazon, its easy to identify the "Grand Textbook for People Taking Multiple Years of This With Professors, TAs, and Problem Sets" and the "Kind of Trippy Pop Science Book That A Professor Wrote So He Could Get On The Daily Show", but what is in between? I took a lot of math, hard science, and engineering in undergrad, but no physics beyond classical mechanics and no chem so I don't know any of this stuff. I don't want to read a book thats like "Did you know that Quantum Mechanics begins with a 'Q'? That is a very unusual letter, my friends!" but I also don't want to read a book thats like "Page 1: As you can clearly see from the following Lagrangian Chapters 1-10 as listed in the table of contents can be left as an exercise to the reader so your authors have decided to proceed from the beginning of Chapter 11. More specifically, its the eighth page of Chapter 11 so it begins mid-sentence."

Listen to me kid, this is important...

bonobothegreat — Fri, 18 Sep 2009 16:53:17 -0800

"There are only two types of motion in the universe: oscillation or rotation and you can't have exclusively one without a trace of the other. That's the reason machines wear out." Does this statement statement have any basis in fact and can you tell me where it might have originated? I heard that from an in-law's father many years ago. He was an old, old guy who'd been a car mechanic and blacksmith. He said it with the sort of intensity that people usually reserve for things they've cooked up themselves but I always wondered if it was original.

Vacuum Genesis Afterlife?

Baby_Balrog — Sat, 12 Sep 2009 23:03:17 -0800

I'm trying to explain vacuum genesis and the probability that we'll all appear here again at some point.... to some friends. We've all been drinking. I know I saw an article where a physicist - a woman - stated something along the lines of - "I know I'll be here again."

I'm trying to explain how particles appear in a vacuum, how this is testable, how given a long enough amount of time and a large enough vacuum that anything might appear. Including an awesome world where we all exist again and have magical powers. And I have a pet dragon.

Where is the article I'm thinking of? I know it had a picture of Mozart's bust and it was green... because a green bust of Mozart will bamf into existence at some point in the impossibly far future.

Halp. They don't believe me.

Would someone die a gruesome death?

ThomThomThomThom — Thu, 10 Sep 2009 08:17:27 -0800

A handstand on someone's chest while lying on a bed of nails? Ok, so this came up in conversation with a street performer friend of mine. It's been a while since I've done any physics and figured that some of you might have been wondering the same as myself. So I am inclined to ask, dear AskMe! (In case you're wondering, I'm not planning on doing this. I'm just trying to settle an idle curiosity.)

Imagine, if you will, a young so-and-so lying on his back on a bed of nails. This bed of nails (as most do) extends the length of man's back - from the lower back, just above his buttocks, to his shoulders and the base of his neck. The nails' heads are approximately 1mm^2 in area at the point (we're talking longish ten-penny nails.) These nails are spread one per square inch on the bed (as are most standard beds-of-nails.) This fellow in particular weighs about 160lb, as does his friend, who now places his hands on the first young daredevil's chest and presses up into a handstand. Let's also assume that these young men (or women!) are shirtless. It's difficult to imagine people being so daring with clothing on.

Now for the million dollar question - did the first performer just meet a gruesome death? I imagine that the jolt of the handstand-er pressing up onto his friend's chest with his hands would add a considerable amount more force than would his simple bodyweight and gravity. Would that be enough to impale his friend? If that would be, what would the effect be like if the man on the bed of nails were to support the handstander's shoulders with his arms, while the handstand-er used his arms to brace himself on the lying-down man's knees? (knees which are connected to his legs, which are pressing against the ground, not touching any nails at all. From what I've researched, this pose is called the candlestick.) Would that take enough force away from the man's back that he wouldn't get impaled?

Thanks for humoring me, dear Metafilter. Inquiring minds simply *have* to know.

Condensed Matter Forum?

Aanidaani — Mon, 17 Aug 2009 10:20:27 -0800

I'm looking for a forum that hosts discussion in the field of condensed matter physics and/or materials science. I'd like a site where scientists can go to ask and answer questions, similar to Ask.Metafilter, but a general forum would be good as well. Does anyone know of such a site?

Does one 200 watt bulb put out as much or more light than ten 20 watt bulbs?

leemajors — Fri, 14 Aug 2009 02:26:17 -0800

Does one 200 watt bulb put out as much or more light than ten 20 watt bulbs? We were sitting in a bar the other day and there was a lamp on the table with a 200 watt bulb. Above us, were lots of lamps, each one considerably less bright than the lamp on our table but it got us to thinking: would the collection of lamps above us put out the same amount of light as the one lamp on our table? I

Are satellites built from old metals to avoid post-A-bomb radiation levels?

metaBugs — Tue, 04 Aug 2009 09:49:45 -0800

Are satellites (or satellite parts) ever made from metal salvaged from pre-1945 shipwrecks? Possibly to ensure that those metals were refined before atmospheric A-bomb explosions increased the background radiation levels? A friend of mine recently visited the ESA site (Europe's answer to NASA) in the Netherlands. While there, he was told by one of the guides that some satellite components are made from metals salvaged from old (WW2 or earlier) shipwrecks. The reasoning ran:

i) The planet's atmospheric background radiation is measurably higher now than it was before the A-bomb detonations.
ii) Therefore, some materials manufactured before the war (presumably referring to steel here) have a lower background radiation than the equivalents manufactured today.
iii) For some scientific applications, having slightly less radioactive materials is worth the extra expense of hauling old shipwrecks up from the seabed and re-processing the materials. Examples given were very sensitive radiation sensors and building radiation shields around delicate equipment.

I've heard this before from another source, although can't remember what that source was. I can just about imagine a mechanism (an increased proportion of Carbon-14 in the Carbon used to make steel from iron?), and Europe certainly has enough WW2-era shipwrecks available for this to be logistically possible.
However, a phycisit friend of ours reckons that this is probably nonsense. She argues that radiation isn't contagious; there's no way a slightly increased atmospheric radiation level could affect the radioactivity of a newly refined slab of steel.
So my questions are:

1) Have you heard that satellites or their components are sometimes made with metal from shipwrecks? What's your source for this?
2) If satellite parts are made from shipwrecks, could it be due to different radiation levels? If so, why is old metal less radioactive?
Bonus question:
3) Is my hypothesis about increased levels of Carbon-14 in modern steel completely insane? If not, could an archaeologist in the future apply Carbon-14 dating techniques to chunks of steel in the same way that we do to our archaeological finds?

Under Pressure

smoke — Mon, 03 Aug 2009 17:29:39 -0800

How would you get through a black hole? I've been thinking about this one for a while, I've found some info about the possibility but not anything that gets into the mechanics.

Say we did find one of these "milder" singularities - presumably the gravity would still be almost neutron star crushing levels, what kind of ship/situation would we need to get through this? I was thinking maybe some kind of oxygenated liquid to inhale and fill the ship?

I have no idea. My physics knowledge is effectively nil. Help me, Mefites, you're my only hope!

How was it proved that mass exerts the gravitic forces?

Lorc — Mon, 06 Jul 2009 11:00:14 -0800

How was it proved that gravitation was a function of mass? If we can estimate planetary masses from the strength of their gravity, including that of the earth, then we must be pretty confident that we've nailed the relationship between the two.

I know that the force of gravity is proportional to the mass of an object and inversely proportional to the distance. And certainly when we look at the universe there's evidence of strong gravity wherever we see a lot of mass. But I also know that science is robust and does not just accept obvious observations without testing them to destruction. How do we know that there's a causal relationship rather than it being circumstantial? And that it exists at all scales, not just with planetary masses?

It seems to me that you'd have to test known masses and be able to measure that object's gravity in order to prove anything. Is that what they did, and if so, how was it done? Is there a gravitational equivalent of Rutherford's gold foil and Robert Millikan's oil-drops?

Bill Nye should be a Mefite.

Christ, what an asshole — Mon, 06 Jul 2009 07:13:46 -0800

What are some simple experiments that help explain complicated phenomena? I was at a geology museum last year, and they had an exhibit where water slowly trickled down an embankment of sand. The path the water took was a seemingly random assortment of forks and zigzags. The accompanying placard read, "This is how Earth's rivers were formed over millions of years." The kid in me just went, "OHHHH!!"

When I was 7, my older brother challenged me to a bet of what would hit the ground first if dropped from the same height (a tie would mean I was wrong): a sandbag or a pen cap. I lost a dollar. Then two. Three. Then he offered to let me try. Four dollars. Five. I quit in frustration. Then came back when he left and kept trying for a half hour, and I thought I was going insane. When 3rd grade rolled around and we began learning about gravity in science class, I was well ahead of the curve, $5 poorer yet $5 wiser.

During Seder this year, my cousins were presented with the Monty Hall problem, and after at least 30 mins. of discussion, they still would not budge on the odds being 50/50. So I came up with this experiment. "I'm going to write down a number between 1 and 1 million. Try to guess it. Can we all agree that you have a 1 in a million chance of getting it right?" (Yes.) The guess was 23. "Alright, I'm going to eliminate numbers 1 through 22, 24 through 625,624, and numbers 625,626 through 1 million. Now would you like to stick with your original guess, 23, or switch your answer to 625,625?" Their eyes lit up. Well, at least three of them. My fourth cousin stubbornly stood by 23. I challenged her to try being the "host," and halfway through, it clicked, and she started damning herself.

What other interesting concepts or phenomena can be broken down in simple terms/experimentation?