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Comment Re:Bullets don't knock people down (Score 4, Insightful) 369

(Trigger warning: Physics teacher here!)

Actually, momentum is NOT conserved for the *shooter*. He is bracing himself against something (his feet on the ground, usually), so there is an external force acting on him. Thus, this particular argument fails.

For the victim, if he is shot unawares (so he doesn't brace himself), then conservation of momentum *does* apply. I calculate that he will experience a force to his chest of about 35 newtons (~8 pounds). That's not much, and won't accelerate him much.

However--it may well knock him down. The reason is, the force to his chest will cause a torque on him, which will cause him to rotate down to the ground. If we assume he rotates about his feet, and treat him as a solid cylinder (reasonable approximation) then I get an angular acceleration of about 0.4 radians per second per second (22 degrees per second per second).

That only lasts while the bullet is in contact with him, of course; after that, the victim has a gravitational torque on him, with a corresponding angular acceleration (I estimate) of 9 rad/s/s.

If someone wants to check my work, I'll supply the numbers and things I used. I might well have made a mistake or missed something.

Comment Ikea Bekant (Score 2) 340

I got the Ikea Bekant; it's great, solidly built, and less than $500US. I used a felt pen on the legs to mark my optimum sitting and standing heights so that I don't have to take five minutes fine-tuning the height every time I change heights (which would have guaranteed that I would never change the height!).

Sometimes I change multiple times a week, sometimes multiple times a day. It's great.

Comment DIY Speakers (Score 4, Interesting) 249

I obsess, but don't spend $$$$ on equipment. As someone posted above, once you have equipment that's better than "on sale at Walmar!", most people won't hear any difference between amplifiers, or CD players, or the audio from Blu-ray players. (You can in a side-by-side A/B test, but what fraction of people do that in their homes?)

My obsession, and my money, went to making my listening room/home theater as acoustically perfect as the floorplan of my house would allow. I put up walls to prevent most standing waves, acoustically treated the walls and ceiling (all in consultation with a professional acoustician; I didn't just stick stuff up on the walls randomly), and positioning my speakers as optimally as possible. Made a WORLD of difference in the sound quality.

And, the other really great investment I made was building my own speakers. Designs exists on the Intertubes (e.g., and the enormous fun and satisfaction of building them yourself cannot be overexpressed. Plus--with all due modesty--my speakers sound as good as speakers that cost 10-50 times what I spent (not including tools, heh). Friends that hear them are astonished that they are DIY speakers.

(N.B.: I am not taking credit for the designs; I have neither the knowledge nor the skills to design quality speakers. I simply implemented the designs and plans I found. There are some seriously smart people out on the web.)

Submission + - Every Time You Fly, You Trash The Planet — And There's No Easy Fix (

jIyajbe writes: Christie Awchwanden of writes "When the latest international Climate Conference wrapped up in Lima, Peru, last month, delegates boarded their flights home without much official discussion of how the planes that shuttled them to the meeting had altered the climate. Aircraft currently contribute about 2.5 percent of global carbon dioxide emissions...if the aviation industry were a country, it would be one of the world’s top 10 emitters of CO2. And its emissions are projected to grow between two and four times by 2050."

Beyond just volume of emissions:

Planes don’t just release carbon dioxide, they also emit nitrogen oxides, sulphur oxides and black carbon, as well as water vapor that can form heat-trapping clouds...These emissions take place in the upper troposphere, where their effects are magnified. When this so-called radiative forcing effect is taken into account, aviation emissions produce about 2.7 times the warming effects of CO2 alone...

Submission + - MIT Removes Online Physics Lectures and Courses by Walter Lewin (

jIyajbe writes: MIT is indefinitely removing retired physics faculty member Walter Lewin’s online lectures from MIT OpenCourseWare and online MITx courses from edX, the online learning platform co-founded by MIT, following a determination that Dr. Lewin engaged in online sexual harassment in violation of MIT policies.

Comment Brick and Mortar (Score 2) 131

If there is a decent brick-and-mortar store within a reasonable driving distance, I'd shop there. Of course, this won't change what quality batteries you find, but--Apart from the issue of supporting your local economy, if the battery fails within the warranty period, it's a heckuva lot easier to return/replace/exchange it than trying to fiddle with an online retailer.

Plus, some stores (my local Batteries and Bulbs store, for example) can open many devices that are not designed to be opened by the average consumer. Finally, they are a LOT more likely to sell you the correct one on the first try.

Submission + - Nightfall: Can Kalgash Exist? (

jIyajbe writes: Two researchers from the Indian Institute of Astrophysics investigate the imaginary world of Kalgash, a planetary system based on the novel ‘Nightfall’ (Asimov & Silverberg, 1991). From the arXiv paper:

"The system consists of a planet, a moon and an astonishing six suns. The six stars cause the wider universe to be invisible to the inhabitants of the planet. The author explores the consequences of an eclipse and the resulting darkness which the Kalgash people experience for the first time. Our task is to verify if this system is feasible, from the duration of the eclipse, the ‘invisibility’ of the universe to the complex orbital dynamics."

Their conclusion?

"We have explored several aspects of Asimov’s novel. We have found that the suns, especially Dovim are bright enough to blot out the stars. Kalgash 2 can eclipse Dovim for a period of 9 hours. We also tested one possible star configuration and after running some simulations, we found that the system is possible for short periods of time."

Comment Re:Streaming vs downloading (Score 1) 339

And in fact, when I buy a movie via iTunes, I have the choice of streaming it directly from Apple servers, or downloading it to my computer--or both. I generally choose to stream directly, because I'm lucky enough to have an internet connection that is both fast and reliable, and Apple's servers have been very reliable (not 100%, but close enough), so I don't have to use up my hard drive space.

But, if I choose to, I can have the full HD version on my local hard drive, and so can watch any movie I buy whether or not I have an internet connection.

(N.B.: I know I sound like an Apple fanboi (TM), but it's the only online service I've tried.)

Comment Re:Avoiding Re-Buying (Score 0) 339

Well, Apple didn't make me pay for the upgrade from Mountain Lion to Mavericks...

They didn't make me pay for the upgrade from iWork '09 to iWork 2013...

They didn't make me pay for some music I bought that was re-issued as remastered shortly after I bought it...

However, you're quite right about having to buy new equipment. That would be true regardless of whether I stream or use physical discs, though.

Comment Only Relevant to Projection (Score 4, Informative) 261

In a movie theater, which uses projection, the curved screen is to alleviate the pincushion effect ( created by the anamorphic lens ( that the theater uses. This is utterly irrelevant to the image created by a monitor TV.

In short, yes; pure marketing BS.

Comment Avoiding Re-Buying (Score 0) 339

I decided to move completely to streaming (via iTunes), rather than buying discs. Two advantages:

1) When the next format comes out, I don't have to re-purchase the same movie yet again (VHS --> DVD --> Blu-ray --> 4K --> ???)

2) Less stuff in my house. I am trying to downsize, and I want my next house to be less than 1000 ft^2. The less stuff I have to store, the better.

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