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Andy Ruina
Attends Cornell University
Lives in Ithaca, New York
55 followers|11,460 views
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Andy Ruina

commented on a video on YouTube.
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Destin:  I think your eyes are more developed than mine.  I am sure you see something at 250K/s that I don't see.  May I suggest that at the highest frame rate you do at least one play back at about 2 frames/s?  Please.  Unfortunately I can't get YouTube to play back like that for me.    Thanks.
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Andy Ruina

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Hi Dan:  
  I woke up this morning and wondered about you.  I found some of your your nice instructional videos. A bit RIBS like somehow! The monkey video is really good.    I think I may know where is an unused high-quality microphone (I gave one to my sister because of her unfulfilled audio-production ambitions). Still interested in a good mike?
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Andy Ruina

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Too fast!  Slow it down more.  Or go frame by frame at 25K/s.   How does the needle prick turn into the circumferential crack? How does that open up and leave the balloon shaped water blob?
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You don't need to slow it down anymore to answer either of those questions. Once the needle has puncture the rubber, the weight of the water in the bottom of the balloons pulls on the rubber, causing it to tear horizontally. Not to mention, it's easier for the tear to travel along where the stress in the rubber is the same, rather than traveling down or up into areas where the rubber is stretched different amounts. The balloon shaped water blob is left simply because the rubber retracts faster than gravity can pull the water down. I don't think slowing it down more would have answered either of those, since those were, in a sense, intuitive.
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Andy Ruina

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Perfect video and explanation.  A real public service.
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Andy Ruina

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Thanks so much for sharing this!
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Andy Ruina

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Watching anything move in zero-gravity is fun.  But I find this explanation of the gyro a big off.  The part where he tapes gyros together.  He seems to imply that by having three gyros pointing in three directions that he is somehow covering 3 components of some vector.  Rather, instead, he just gets a new angular momentum vector pointed along the diagonal with a magnitude  sqrt(3) larger.  

What would a viewer of this video think would happen if he taped two spinning players back to back?  I guess that most would not predict that the gyro effects would cancel and the CDs would tumble in space the same way whether they were spinning or not. I think that the explanation in the video about taping the CDs together, leads one to vague and wrong notions about the vectorial nature of angular momentum.

Reverse-engineering the astronaut's mind further, I think that he thinks that the angular momentum vector is made up of three independent positive components which have independent effects.   And he thinks he can manipulate those components separately.  So he thinks he stabilizes one axis at a time.  Rather, each time he adds a CD he is changing the magnitude and direction of the net angular momentum vector.  His final product  hasn't "covered" three dimensions. It has instead just covered a single crooked direction a bit (sqrt(3)) better. 
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Have him in circles
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Andy Ruina

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Want to share your location with me on Google+? Turn it on here: https://plus.google.com/settings
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Andy Ruina

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The boat is too beautiful.  Can I stop by Thursday or Friday to chat with you guys?
I am still interested in the minimum-rudder-use self-stable concept.
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Andy Ruina's profile photoRonny Eriksson's profile photo
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Welcome!
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Andy Ruina

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I think the explanation of Newton's third law  here  is great, in terms of its applicability to gravity.  But the symmetry argument invoked doesn't apply in all kinds of situations where the third law does apply.  Like the friction force between dissimilar materials. 
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Andy Ruina

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I think Lucian needs one of those balls in school.  Maybe he could stop his ritalin.
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Andy Ruina

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Hey man.  I've done my part.  But I am just one person.  So how am I supposed to find out how to make a balloon quiet??  -Andy Ruina (ruina@cornell.edu).
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Andy Ruina

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People
Have him in circles
55 people
Kiisa Nishikawa's profile photo
Jungkeun Bae (jumiss)'s profile photo
Dirk van Nouhuys's profile photo
Elisabeth Iler's profile photo
Gregg Stiesberg's profile photo
David Smullin's profile photo
Noopi Grewal's profile photo
Jason Moore's profile photo
Arend Schwab's profile photo
Work
Occupation
Professor, scientist
Employment
  • Professor, scientist, present
  • USGS, Shell, Texas Instruments, Appel Farm
Places
Map of the places this user has livedMap of the places this user has livedMap of the places this user has lived
Currently
Ithaca, New York
Previously
Providence, Cambridge, Boston, Washington, Arlington, Champaign Lexington, Chevy Chase - Menlo Park, Finland, Åland, Palo Alto, Israel
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Story
Introduction
Born 1953 in Providence. Moved to Champaign IL, Alexandria VA, Arlington VA, Lexington MA, Chevy Chase MD  and Cambridge, MA. Spent a year in Isael.  Went to Brown for 3 degrees. Studied, mechanics, friction and earthquakes.  Taught at Cornell in Theoretical and Applied Mechanics since Aug 25, 1980. Maried to Saskya van Nouhuys since May 15, 1993.  Two daughters: Mieke (born 1993) & Prachi (born 2001).
Education
  • Cornell University
    present
  • Cambridge School of Weston, Brown, Sidwell Friends, Taylor Elementary, George Mason Green,
Basic Information
Gender
Male
Other names
Andrew
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It's good food. It's made with fresh vegetables. It's not authentic Mexican. But it doesn't claim to be. Whole meal with drinks is about 15 dollars.
Food: ExcellentDecor: Very GoodService: Very Good
Public - 2 years ago
reviewed 2 years ago
Now the home of Andy Ruina, Saskya van Nouhuuys, Prachi Ruina and Mieke Ruina. Kim Karlsson doesn't live here anymore. The address is also called Skedholmsvägen 17.
Public - 2 years ago
reviewed 2 years ago
2 reviews
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