The Science Thread
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Topic: The Science Thread
Posted By: King of Loss
Subject: The Science Thread
Date Posted: December 24 2013 at 15:34
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Hey guys, so I don't see a science thread, so I'd like to start one.
What interests me these days is the advent of rocket-technology and ways to harness it. A quick video on it: More on the Sabre rocket engine: http://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29" rel="nofollow - http://en.wikipedia.org/wiki/SABRE_(rocket_engine)
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Replies:
Posted By: dr wu23
Date Posted: December 24 2013 at 16:38
Hmm...I see we are going to need some...."heh, this isn't rocket science'...jokes.
 ------------- One does nothing yet nothing is left undone. Haquin |
Posted By: King of Loss
Date Posted: December 24 2013 at 18:05
 Well, science jokes also apply.   |
dr wu23 wrote:Posted By: Slartibartfast
Date Posted: December 24 2013 at 18:13
And of course we can't leave out Thomas Dolby. 
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Posted By: King of Loss
Date Posted: December 24 2013 at 18:54
Thomas Dolby?!?!?!
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Posted By: Slartibartfast
Date Posted: December 24 2013 at 19:03
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Ray Dolby was a mere piker compared to Fred DBX.  |
Posted By: infocat
Date Posted: December 24 2013 at 19:10
 Please continue! ------------- -- Frank Swarbrick Belief is not Truth. |
Posted By: Slartibartfast
Date Posted: December 24 2013 at 19:15
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String theory, anyone?
------------- Released date are often when it it impacted you but recorded dates are when it really happened... 
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Posted By: infocat
Date Posted: December 24 2013 at 19:36
 Haha, sorry. ------------- -- Frank Swarbrick Belief is not Truth. |
Posted By: King of Loss
Date Posted: December 24 2013 at 19:46
| A serious thread, guys.... |
Posted By: Dean
Date Posted: December 25 2013 at 03:18
My favourite Facebook page is this one: https://www.facebook.com/IFeakingLoveScience" rel="nofollow - https://www.facebook.com/IFeakingLoveScience
 ------------- What? |
Posted By: Toaster Mantis
Date Posted: December 25 2013 at 04:00
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I guess this is as good a time as any to plug one of my favourite bloggers, Scott Locklin who just wrote http://scottlocklin.wordpress.com/2013/11/14/the-most-beautiful-rocket/" rel="nofollow - a very good post on the state of the US space program. He sounds quite dispirited at how slow progress is going compared to the Cold War days, with more innovative and ambitious projects being canned to save money. ------------- "The past is not some static being, it is not a previous present, nor a present that has passed away; the past has its own dynamic being which is constantly renewed and renewing." - Claire Colebrook |
Posted By: Icarium
Date Posted: December 25 2013 at 13:04
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I would not mind seeing Venus so close, would be liek a Twin planet thingy, and two objects were life could thrive, but then both planets might would orbit each-other, in a very cool way. though high tide and low tide water would be little more extreme -------------  
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Posted By: Icarium
Date Posted: December 25 2013 at 17:29
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this made my brain hurt -------------
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Posted By: The Pessimist
Date Posted: December 25 2013 at 18:42
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A photon checks into a hotel. The bellhop asks "Can I help you with your luggage?" To which the photon responds "No thanks, I'm traveling light."
------------- "Market value is irrelevant to intrinsic value." Arnold Schoenberg |
Posted By: King of Loss
Date Posted: December 25 2013 at 20:20
Maybe you can watch it in short bursts. My dad is a neuroscientist, so I get a lot of this all the time.
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Posted By: King of Loss
Date Posted: December 25 2013 at 20:27
This is what happens when you allocate most of your money in maintaining such a large military empire... You forget to utilize it for tools that actually benefit mankind.
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Posted By: Toaster Mantis
Date Posted: December 26 2013 at 04:32
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Thing is, that actually happened during the Cold War with so much of its landmark technological advancement both civilian and military being rooted in the arms race. Then again, maybe your point is that with people being stuck in that mentality it's difficult to build up much motivation to ambitious aerospace development in the US now that they don't have an evenly-matched enemy. (though Vladimir Putin might be working on changing that...)
------------- "The past is not some static being, it is not a previous present, nor a present that has passed away; the past has its own dynamic being which is constantly renewed and renewing." - Claire Colebrook |
Posted By: Equality 7-2521
Date Posted: December 26 2013 at 10:54
Probably because some of it is false and the rest seems to be purposely obfuscated for what appears in intent to be a pop-sci video. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Icarium
Date Posted: December 26 2013 at 11:56
neurology is more scientific valuable then psychology -------------
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Posted By: Toaster Mantis
Date Posted: December 27 2013 at 04:54
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Not so fast bucko. I've actually written a term paper at college a couple years ago about the limits of physicalism, and back then the actually available evidence suggested that the identity between physical conditions in the brain and mental states is nowhere as clear-cut as certain people like to suggest. http://www.spiked-online.com/newsite/article/putting_the_human_soul_on_the_slab/14338#.Ur1bNLTzV8k" rel="nofollow - This article suggests it's still as controversial, as does the rest of the debate about the ethics of psychiatric medication which has been raging very strongly in the UK media recently. ------------- "The past is not some static being, it is not a previous present, nor a present that has passed away; the past has its own dynamic being which is constantly renewed and renewing." - Claire Colebrook |
Posted By: Equality 7-2521
Date Posted: December 27 2013 at 07:24
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That article suggests nothing. There's nothing scientific in it. A quote from a philosopher who died long before the field of neuroscience existed properly marks the closest that it came to presenting any source for the claims made.
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: January 06 2014 at 08:35
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http://www.nytimes.com/2014/01/05/sunday-review/why-everyone-seems-to-have-cancer.html?smid=tw-share&_r=0" rel="nofollow - Why Everyone Seems To Have Cancer ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: January 09 2014 at 09:56
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I showed this video to my students this morning to teach a few meta-mathematical things and give them an example of a series. http://www.youtube.com/watch?v=w-I6XTVZXww" rel="nofollow - 1+2+3+...=-1/12 It garnered more interest than I expected so I thought I'd post it here. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Padraic
Date Posted: January 09 2014 at 10:50
Very cool, interesting paradox which starts with the Grandi's series (1, -1, 1, ...). Although I don't like the manipulations that led to it's sum = 1/2.
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Posted By: Icarium
Date Posted: January 09 2014 at 10:50
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Its the ofdest time sig coveyed in music -------------
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Posted By: Dean
Date Posted: January 09 2014 at 11:05
He could have done S-(-S) = 2S = 1 1-1+1-1+... and 1+2+3+4+... are still a divergent series. ------------- What? |
Posted By: Padraic
Date Posted: January 09 2014 at 11:07
That's what he did do. And indeed they are.
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Posted By: Dean
Date Posted: January 09 2014 at 11:19
Yeah, I meant he could have shown that's what he did, he just said it averages at ½. ------------- What? |
Posted By: Padraic
Date Posted: January 09 2014 at 11:20
| I'd be interested in hearing the dialogue that Pat had with his students about it. |
Posted By: Equality 7-2521
Date Posted: January 09 2014 at 12:14
The manipulation is not valid. It's the right intuition that would lead you to proving the result rigorously using Ramanujan summation. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: January 09 2014 at 12:17
I'm not positive that I'm eloquent enough for it to have been that interesting. I tried to remind them about the dangers of conflating the colloquial meaning of words and the technical meaning of words ('the sum is'), how intuition and specifically usual arithmetic manipulations do not carry over to the infinite, and the fact that even really silly and recreational looking things in math tend to have profound ramifications on our world. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: January 09 2014 at 12:26
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Pat: An elementary and sound explanation is given http://math.ucr.edu/home/baez/qg-winter2004/zeta.pdf" rel="nofollow - here if you would like to read it.
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Padraic
Date Posted: January 09 2014 at 12:30
Maybe it's the infinite part, but: aren't the natural numbers closed under addition?
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Posted By: Equality 7-2521
Date Posted: January 09 2014 at 12:37
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They are. However, addition is only a binary operation +: N x N -> N Using the inductive principle, you can easily extend this to an operation on any finite number of inputs because of associativity. However, it cannot be extended to the infinite. With usual infinite summation, you're considering a limit of finite sums which can leave your closed Ring. This is how we deal with real numbers: as the limit of a sequence of rationals. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Padraic
Date Posted: January 09 2014 at 14:58
Ha! This quote perfectly expresses how I feel about this! "The divergent series are the invention of the devil, and it is a shame to base on them any demonstration whatsoever." |
Posted By: Equality 7-2521
Date Posted: January 09 2014 at 15:00
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Old mathematicians were pithy as hell.
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: King of Loss
Date Posted: January 09 2014 at 23:08
Maybe you can explain that mathematical effect to our politicians.
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Posted By: King of Loss
Date Posted: January 09 2014 at 23:17
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I found this video watching Pat's video on the sum of 1+2+3+4+5+….=-1/12, I have found this video: I think it's quite interesting because mathematics also affects our personal privacy! |
Posted By: A Person
Date Posted: January 10 2014 at 00:23
| I just started a course on cryptography on Coursera. It turns out cryptography is difficult. |
Posted By: Equality 7-2521
Date Posted: January 10 2014 at 12:00
Mathematics is our personal privacy in a digital age like this. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: January 10 2014 at 12:52
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http://www.nature.com/news/video-fish-leaps-to-catch-birds-on-the-wing-1.14496" rel="nofollow - Cryptozology finds some scientific corroboration.
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: King of Loss
Date Posted: January 10 2014 at 14:28
Mhmm.. It's very true. It's hard for most people to get that.. Maybe why we're heading towards a lovely serfdom-like existence.
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Posted By: Equality 7-2521
Date Posted: January 10 2014 at 15:18
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I don't think it's hard for people to get. They just don't know much about how their data is kept secure, and they don't bother to find out.
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: January 13 2014 at 11:19
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This is really, really, really cool. Like the coolest thing you'll read this week probably. http://www.upenn.edu/pennnews/news/penn-research-helps-lay-out-theory-metamaterials-act-analog-computer" rel="nofollow - UPenn creates metamaterial which performs calculus. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Slartibartfast
Date Posted: January 13 2014 at 11:31
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Thank gawd for that, I hated calculus in college.
------------- Released date are often when it it impacted you but recorded dates are when it really happened...
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Posted By: Padraic
Date Posted: January 13 2014 at 12:05
I had Engheta for an E&M course. Very cool guy and great teacher.
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Posted By: Dean
Date Posted: January 13 2014 at 12:57
Theoretical material, 'creates' isn't the word I'd use. Analogue computers are cool anyway. It will be interesting to see if they can perform a derivative function in light because it is impractical in electronics - analogue computers could only integrate. ------------- What? |
Posted By: Equality 7-2521
Date Posted: January 13 2014 at 14:42
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^Let's say they've metacreated the metamaterial.
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: King of Loss
Date Posted: January 13 2014 at 15:21
Very very cool. Reminds me of what my grandfather liked to do, except with evil central banking.
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Posted By: Equality 7-2521
Date Posted: January 15 2014 at 15:36
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So that didn't take long, but physicists have figured out the chain fountain problem. The video is quite good and accessible to children The pop-sci article is http://www.nature.com/news/physicists-explain-gravity-defying-chain-trick-1.14523" rel="nofollow - here . The actual paper can be read by a bright high school student and is http://rspa.royalsocietypublishing.org/content/470/2163/20130689.full" rel="nofollow - here . ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: January 31 2014 at 15:50
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http://www.huffingtonpost.com/science-channel/the-top-10-science-jokes-_b_4702239.html?utm_hp_ref=technology&ir=Technology" rel="nofollow - Stephen Hawking tells science jokes
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: March 14 2014 at 13:44
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http://www.youtube.com/watch?v=E36qMxXGo3A" rel="nofollow - A prog song for Pi Day
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: King of Loss
Date Posted: March 14 2014 at 13:49
This sounds like cheesy neo-prog. They should have made a more heavier version.
like Meshuggah! |
Posted By: Equality 7-2521
Date Posted: March 14 2014 at 13:50
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Form over function.
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: The T
Date Posted: March 14 2014 at 15:09
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Can science explain Can? Kayo Dot? ------------- 
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Posted By: King of Loss
Date Posted: March 14 2014 at 15:15
Maybe Kayo Dot, but science cannot explain the awesomeness of Damo Suzuki.
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Posted By: Man With Hat
Date Posted: March 14 2014 at 23:36
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Anyone here interested in the rebooted Cosmos tv series? ------------- Dig me...But don't...Bury me I'm running still, I shall until, one day, I hope that I'll arrive Warning: Listening to jazz excessively can cause a laxative effect. |
Posted By: Dean
Date Posted: March 15 2014 at 04:17
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I haven't seen it (yet). I suspect that people will attempt to compare it with the original, or more likely, the current popular science presenter (no idea who he is¹) to Carl Sagan. But the burning question for me would be: is the music any good? I enjoyed the soundtrack to the original series immensely. ¹ - I know he is Neil deGrasse Tyson, but in the UK Professor Brian Cox is the ubiquitous go-to tv personality on cosmology so Tyson doesn't get a look-in. ------------- What? |
Posted By: Dean
Date Posted: March 15 2014 at 04:20
------------- What? |
Posted By: Atavachron
Date Posted: March 15 2014 at 04:22
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I caught a few minutes and it seemed interesting but I didn't have the patience for the debut, too talky and backgroundy. Maybe it'll liven up as they progress. |
Posted By: infocat
Date Posted: March 15 2014 at 07:50
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I was underwhelmed by the new Cosmos. I like NdT (Stephen Colbert's favorite guest), and I'm not particularly knowledgeable about science, but it seemed a bit too 'dumbed down' (not really the phrase I'm looking for, but I can't think of another). Hopefully will get better. ------------- -- Frank Swarbrick Belief is not Truth. |
Posted By: Equality 7-2521
Date Posted: April 08 2014 at 08:55
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Cool model of a geometric vanish puzzle ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Equality 7-2521
Date Posted: May 21 2014 at 10:20
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http://news.sciencemag.org/social-sciences/2014/05/replication-effort-provokes-praise-and-bullying-charges" rel="nofollow - Need for Replication Is Not Censorship; It's Science
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: King of Loss
Date Posted: May 21 2014 at 21:11
That is very cool!
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Posted By: King of Loss
Date Posted: July 20 2014 at 19:53
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This is quote old news, but bionic arms, legs and other body parts are on the market! Maybe we'll see R2D2 in our lifetimes! http://www.youtube.com/watch?v=CDsNZJTWw0w" rel="nofollow - http://www.youtube.com/watch?v=CDsNZJTWw0w
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Posted By: Equality 7-2521
Date Posted: August 06 2014 at 19:28
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http://medium.com/starts-with-a-bang/how-to-fool-the-world-with-bad-science-7a9318dd1ae6" rel="nofollow - The Miracle Space Drive that Wasn't
------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Gerinski
Date Posted: August 13 2014 at 06:15
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What will happen? Obviously the two containers are exactly the same, with exactly the same volume of water, and the two balls have exactly the same volume.  |
Posted By: Equality 7-2521
Date Posted: August 13 2014 at 07:11
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I'm not sure if you're asking or this is supposed to be a puzzle. So my answer is in white text below: Assuming the strings are mass less, I would imagine that the right hand side would fall. It should be independent on the weight of the two balls, only depending on their volume and the fact that the left hand side will float while the right hand side will not. Essentially what's pressing on the scales on both sides is the weight of the water and the pressure of the displaced water by the balls. However, on the left we will have an opposing contribution due to the tension in the string since it is connected to the scale. The left hand side should be lighter. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Gerinski
Date Posted: August 13 2014 at 07:22
| ^ it was meant as a puzzle. I found it somewhere and I have to admit that although I think I know the answer, I do not know it for sure. I will for the moment refrain from commenting on your solution to see if others have interesting input, I would say that your outcome seems correct to me but not the reasoning. |
Posted By: someone_else
Date Posted: August 13 2014 at 07:28
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I am not sure either, so I give my comment in background color. Anyone who is interested can select the text (i.e. by pressing Ctrl-A) to read it: On the right hand side, the steel ball and the string that suspends it, do not add weight to the scale because the gravitational force is neutralized. On the left hand scale, the weight of the string and the air inside the ball add a little weight (but there is an upward force inside the ball because its density is less than the density of water), so I guess the left hand side will go down. ------------- 
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Posted By: Gerinski
Date Posted: August 13 2014 at 08:08
| ^ let's add that we can neglect the mass of the strings themselves. |
Posted By: Dean
Date Posted: August 13 2014 at 08:22
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In the interest of fairness I'll not read the hidden explanations until other people have "voted", but I'll simply state: "right-side goes down" and leave it at that for now. ------------- What? |
Posted By: Gerinski
Date Posted: August 13 2014 at 09:43
It would be however nice to know the reasoning behind the opinions.
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Posted By: Dean
Date Posted: August 13 2014 at 09:45
I'll give my reasons when more people have answered, I don't want to sway anyone's thinking (which is also why I haven't peaked at Pat and Robert's explanations). ------------- What? |
Posted By: Equality 7-2521
Date Posted: August 13 2014 at 11:14
This in particular ignores the reciprocal buoyancy force at work on the water. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: timothy leary
Date Posted: August 13 2014 at 11:50
| The scale should balance |
Posted By: Gerinski
Date Posted: August 13 2014 at 11:53
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This is becoming "the invisible ink thread" OK I leave some more time for our friends in other time-zones, tomorrow we can be more explicit about it.
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Posted By: Gerinski
Date Posted: August 14 2014 at 05:22
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^^ OK so here is what I believe is the correct answer, which was also Dean's. The left container forms an isolated system, with no external forces acting on it (other than gravity of course). The left scale plate holds the mass of the container, the mass of the water and the mass of the ping pong ball. On the right side the steel ball is held hanging from outside the container system so its mass is not supported by the scale plate. The scale plate holds the mass of the container, the mass of the water and the buoyant force of the ball, which Archimedes told us is equivalent to the volume of water displaced by the ball. Since a ball of water is heavier than the ping pong ball, the scale will tip to the right. Notice that if the ping pong ball was being pushed and kept submerged from the top from outside the container, say by a wire rod with a small cupped end at the bottom, then the scale would stay balanced. Incidentally, it is incorrect to think that the ping pong ball will exert a force upwards on its container due to its tendency to float. They form part of the same system and as they say, you can not lift yourself up by pulling from your bootstraps.
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Posted By: Dean
Date Posted: August 14 2014 at 05:38
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Looking at the hidden answers Pat gave the same explanation, his last sentence (the left side would be lighter) may have given the wrong impression. ------------- What? |
Posted By: Dean
Date Posted: August 14 2014 at 05:41
Yes, exactly. You could replace the steel ball and string with a ping pong ball and rigid rod and the right-side will still go down. ------------- What? |
Posted By: Gerinski
Date Posted: August 14 2014 at 06:33
His conclusion was correct, the scale will tip to the right, but I'd say that the reasoning was not correct: "Assuming the strings are mass less, I would imagine that the right hand side would fall. It should be independent on the weight of the two balls, only depending on their volume and the fact that the left hand side will float while the right hand side will not. Essentially what's pressing on the scales on both sides is the weight of the water and the pressure of the displaced water by the balls. However, on the left we will have an opposing contribution due to the tension in the string since it is connected to the scale. The left hand side should be lighter. " 1-it is not the fact that one ball floats and the other not which is relevant, as I said the ping pong ball would balance the scale if it was pushed submerged from outside, having exactly the same tendency to float. 2-the displaced water does not contribute to the weight of the left, only on the right. If it did, the scale would stay balanced. 3-the tension in the string does not "reduce the weight of the left side" since they for part of the same system. No matter how big a bag full of air you would submerge (tied to the bottom of the container) it will never lift up its own container.
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Posted By: Equality 7-2521
Date Posted: August 14 2014 at 06:49
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1) I see the tension in the string keeping the ball in equilibrium to be an important part of the analysis. EDIT: Also mean to say, you are correct the displaced water does not factor on the left. EDIT Again: A google image searched turned up a solution where someone was nice enough to draw the diagram and write out the equations. So definitive answer I suppose http://physics.stackexchange.com/questions/130688/which-way-does-the-scale-tip" rel="nofollow - here. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Dean
Date Posted: August 14 2014 at 07:01
As I love to disagree, I shall continue to do so. Both objects have the same tendency to float by the same degree, both are exerting the same downward force on the scale. That is what Pat has said in the second sentence you have highlighted The steel ball does not float because the force of gravity acting upon its mass is far greater than the buoyancy force pushing it up. That is what Pat has said in the first sentence you have highlighted. The tension in the string that keeps the ping pong ball submerged is exerting an equal and opposite force on the ping-pong ball to counter-act the weight of water the ball displaces. That is what he has said in the third highlighted sentence. The left-hand side is lighter than the right, it is not lighter because it is trying to pull itself up by its own bootlaces (which is what you have interpreted).
1. The ping-pong ball floating is wholly relevant because of the tension in the string. The tension of the string counter-acts the buoyancy so the net result is equilibrium. This is the same as taking the ball and string out of the water and placing it alongside the beaker on the scale - only then does "floating" become irrelevant. 2. Correct, but weight is a force. The displaced water contributes equally in both instances but in the left-hand beaker there is an opposing force that negates this increased "weight" 3 - Yes it does. The total "weight" is the mass of the (beaker + water + string + ping-pong-ball) x g plus the buoyancy of the displaced water minus the tension (force) in the string. ...plus whatever Pat wrote while I was typing this.
------------- What? |
Posted By: Gerinski
Date Posted: August 14 2014 at 07:13
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OK, perhaps the wording was not clear enough to me. Here is the actual experiment, long preparation, you can jump directly to the 10 min mark |
Posted By: Dean
Date Posted: August 14 2014 at 08:10
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This was a fun puzzle Gerard. 
The first thing you will notice is the reading on the scales should increase as the object is lowered into the water. Once the object is fully submerged the reading should not change until the object hits the bottom of the jug. What you should find is
from these results:
Because 1ml of water weighs exactly 1gm we can then calculate the predicted weight of the displaced water simply by multiplying the volume (B) by the constant 1 gm/ml, therefore: F: (B) x 1gm/ml from those calculations you should find:
...there will be a slight error in the last one because the graduations on kitchen jugs aren't very accurate compared to the accuracy of the scales. ------------- What? |
Posted By: Equality 7-2521
Date Posted: August 14 2014 at 08:36
You responded on my behalf much more eloquently than I did. Thank you. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Gerinski
Date Posted: August 14 2014 at 08:54
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Alright here's another one, this one is easy but believe me, I have seen a couple of people scratching their heads for a couple of minutes... GRAVITY AT THE CENTER OF THE EARTH (for simplicity we assume that the Earth is perfectly spherical and that at any given distance from the center towards the surface it's average density the same in every direction) Obviously being on the surface, the Earth's gravity pulls us towards its center. Now imagine, hypothetically of course, that you sit at the very center of the Earth. What will you feel? a) since the Earth's mass is now surrounding you in every direction, its gravity will pull you outwards, you will feel stretched out in every direction towards the surface. Since the pull is the same in every direction you will not move anywhere but your body will be stretched outwards. b) gravity pulls you outwards towards the surface, but since the force is the same in every direction they all cancel out so you will not feel anything at all, you will feel weightless but "not stretched". c) even if the Earth's mass is now surrounding you in every direction, you will still be crushed to the very center to a ball the size of a pea if not less
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Posted By: Dean
Date Posted: August 14 2014 at 09:18
My pleasure. As you answered the puzzle first I thought you deserved the credit for giving the correct reason too.  ------------- What? |
Posted By: Dean
Date Posted: August 14 2014 at 09:34
I would feel very hot and short of breath. Other than that, none of the above, but b) is the closest. ------------- What? |
Posted By: Equality 7-2521
Date Posted: August 14 2014 at 11:02
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Yeah with the obvious simplifying assumptions why would anyone think anything except (b). ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Gerinski
Date Posted: August 15 2014 at 07:40
Alright, nobody else posted so I guess that we can crack this one. As is often the case with puzzles, the key to this one lies in that the subject is phrased in a deliberately misleading and incomplete way. At first the question seems to be about gravity, but then what is actually asked is "what will you feel" or "what will happen to you" which is an altogether different question. Without any additional information, and ignoring the effects of the extreme temperature believed to be around 6000 Celsius at the inner core, if you would magically suddenly appear at the center of the Earth you would be immediately crushed into nothing by the huge pressure, even if the net gravitational force there is indeed zero, same as if you dive too deep in the ocean you will be crushed by the hydrostatic pressure even if the gravitational force at that depth is less than at the surface. So the correct answer is "c". Only if we were told that you were sitting at the center inside an hypothetical protective spherical shell capable of supporting the immense pressure and holding atmospheric pressure in its interior, then would "b" be the correct answer. |
Posted By: Dean
Date Posted: August 15 2014 at 07:57
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Oh no. 'Hypothetical' implies that, as Pat said, certain simplifying assumptions are made and the bounds of the answer are contained within the limiting conditions set by the question. The primary assumption inherent in the formulation of the question is that the environmental conditions can, hypothetically, support the subject of the experiment. The thought-experiment is only concerned with the effects of gravity. Under those predefined conditions and implied assumptions (b) gives the correct answer but not the exact reason.
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Posted By: Gerinski
Date Posted: August 15 2014 at 08:04
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"for simplicity we assume that the Earth is perfectly spherical and that at any given distance from the center towards the surface it's average density the same in every direction" Where do the "simplifying assumptions" talk about hyperstrong shells? Why would "c" be listed as an option then?
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Posted By: Dean
Date Posted: August 15 2014 at 08:32
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Because every multiple choice has a silly answer? ------------- What? |
Posted By: Dean
Date Posted: August 15 2014 at 08:36
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Also, if the average density in all directions is the same then you have assumed that pressure is also constant through-out the sphere. The water density at the bottom of the sea is greater than at the surface. ------------- What? |
Posted By: Gerinski
Date Posted: August 15 2014 at 09:04
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The assumption said that "at any given distance from the center towards the surface, the average density is the same in every direction". This is not saying that density is the same at any given distance, but only that there are no noticeable concentrations of mass which could pull you in some direction more than to others, that the Earth is formed of concentrical shells of a certain density each. But unless explicitly stated otherwise, logic says that density increases with depth. At any rate, c'mon don't be a bad loser, this one is not about actual scientific knowledge which I know you have in spades, it is about the tricky phrasing, about spotting it or not. If you spot it then it's as simple as replying something like "net gravity at the center is zero but unless you are shielded from the pressure the answer to "what will happen to you" is (c) = you will be crushed into nothing, which has nothing to do with gravity". In any case I'm curious as to why did you say in your first reply that "none of the above" and in a later one "(b) gives the correct answer but not the exact reason".
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Posted By: Equality 7-2521
Date Posted: August 15 2014 at 11:43
I'm sorry, but no. You can ask hypothetical physical situations for 3 reasons: Pure fun, to clarify understanding of a physical system, to be a smartass. If you're doing the third, then it has no place anywhere really. Phrasing the problem as a scientific question precludes the third. ------------- "One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. " |
Posted By: Dean
Date Posted: August 15 2014 at 12:23
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If you must keep stretching the point about density then you cannot in the same breath ignore the effects of temperature, part of which is a consequence of the pressure, nor can you chose to ignore the composition of the core - when embedded in molten iron you would not survive long enough to feel anything, nor would your body survive long enough to be crushed to the size of a pea. All 'hypothetical' experiments make simplified assumptions. Even with the trick phrasing of "at any given distance" you have made assumptions. As to why "none of the above" and "not the exact reason": the force isn't "the same in all directions"...
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Posted By: Gerinski
Date Posted: August 15 2014 at 13:02
| Alright I see that you didn't like this one, but you can still try it with some of your friends if you want to. |