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  1. J

    I Summation Equality

    Sorry, I discovered my error very shortly after reading Dragon27's reply. I had made a very simple mistake where I wrote \frac{N}{N^2}=N. Just not enough sleep I guess.
  2. J

    I Summation Equality

    I did, but still didn't seem to get it to come out right. But, now that I know it's just me, I'll figure it out. Thanks!
  3. J

    I Summation Equality

    I'm doing my first paper review and an equation is holding me up. I can't tell if I'm just missing something silly or if the author made a mistake. Given that: \sum_{n=1}^{N}s_{n} = 1 The author says that: \sum_{n=1}^{N}(s_{n} - \frac{1}{N})^{2} = \sum_{n=1}^{N}s_{n}^{2} - \frac{1}{N} I seem to...
  4. J

    Electrical DIY Atmospheric Carbon Dioxide Detector?

    Thanks. These are ±70ppm. The sensor I was looking at (MG811 Sensor) is ±50ppm and only $35. So with a homebuilt setup (add on a simple microcontroller and something to power it), I could probably get away with a setup just over $50 (excluding the solar panel cost). If there isn't a better...
  5. J

    Electrical DIY Atmospheric Carbon Dioxide Detector?

    I thought it would be an interesting project to build a detector that will measure and log the outdoor atmospheric CO2 levels. These (global average) levels are currently about 400ppm and these (global average) levels vary by a few ppm every year. Looking for a sensor though, I can't seem to...
  6. J

    How does hot water brew coffee?

    I wondering what actually happens when you brew coffee (or tea) with hot water. Every source I can find simply talks about how long you need to brew it to "draw out" the flavor or "release" the caffeine. I'm interested in what's actually physically happening to the coffee grounds that makes it...
  7. J

    Scientific Politicians In The Midterm Elections

    I want to support a scientifically oriented politician in the upcoming US midterms. I'm not looking to have people argue about which party is more scientifically oriented. Specifically, I want to support someone who either has a primarily scientific background or makes science a top priority...
  8. J

    Are green <5mW laser pointers legal to use in New York City?

    Alright, thanks! I guess I was just being overly cautious and assuming that they were only considering lower power laser pointers here. I'm guessing the sky doesn't count as a public place? I'm still a bit wary that I might be called out as trying to point my laser at planes since there are so...
  9. J

    Are green <5mW laser pointers legal to use in New York City?

    I've been searching for a while, but all I seem to find is information about how it's illegal to sell laser pointers to minors in New York City. That and I find some information on what else is illegal to do with a laser pointer in NYC (pointing at vehicles, officers, etc.). There's a lot of...
  10. J

    Computer Vision - Ground plane position based on image point

    When given an image of a scene of something like a hallway or road (looking down this hallway or road) a vanishing point can be determined. Also, the points on the image must lie on a certain line in the real world environment given by: x_{im} = f\frac{X}{Z} and y_{im} = f\frac{Y}{Z} where...
  11. J

    How does Jupiter protect Earth from comets and such?

    How does Jupiter "protect" Earth from comets and such? I've commonly heard that Jupiter protects Earth by flinging comets out of an orbit that would otherwise leave them heading for Earth. I understand that Jupiter can throw comets and other objects out of their current orbit when they pass by...
  12. J

    Astronomy book listing Ceres as a planet?

    Just in case anyone else would like to know, I found at least one such book: http://archive.org/stream/elementsofastron00vincuoft#page/12/mode/2up/search/Ceres Archive.org apparently has some very good tools for searching. If anyone else would like to find others, simply search for books of the...
  13. J

    Astronomy book listing Ceres as a planet?

    Thanks for the Project Gutenberg, I will give that a try for sure.
  14. J

    Astronomy book listing Ceres as a planet?

    Does anyone know of a (relatively) cheap astronomy book which has Ceres officially labelled as a planet? I realize it may not be extremely cheap seeing that it would have to be from the 1800s, but perhaps there's one that was commonly enough in print to not be so hard to find? It's both...
  15. J

    Asteroid, Meteorite, Meteoroid, or Meteor?

    So I know a meteoroid is the small thing floating in space while an asteroid is the large one. I know a meteor is the streak of light a meteoroid creates as it falls through the atmosphere and a meteorite is what remains after the impact on the Earth. Also, that meteorites can sometimes be...
  16. J

    How to find the plane at which two hyperplanes intersect.

    Of course. You're right on both accounts. Thank you much!
  17. J

    How to find the plane at which two hyperplanes intersect.

    I know that to find the line at which two planes intersect, you can take the cross product of their normal vectors. This gives you a vector parallel to the line. Then you can just find a point which lies on both planes and that position plus the vector is your line. How do you do the...
  18. J

    Having trouble understanding a dimension reduction step in a matrix.

    I'm sorry, when I said "Why are you allowed to do that?" I didn't mean mean the same thing you think I did. Then again, I could have worded that much better. My question is not so much "How would I know when to use that trick?", but instead "What is that trick?". Why are these two mappings...
  19. J

    Having trouble understanding a dimension reduction step in a matrix.

    Yes, that would be a more simple equivalent problem. Again, I can clearly see what's happening, but why are you allowed to do that? Thanks for this much at least.
  20. J

    Having trouble understanding a dimension reduction step in a matrix.

    This is dealing with computer vision, but the only part I'm having trouble understanding is a step in the matrix math. So it seems appropriate it should go here. The paper/chapter I'm reading takes one of those steps saying "from this we can easily derive this" and I'm not quite sure what...
  21. J

    Computer Vision Geometry - Collinear Points In A Pinhole Camera Model

    Like I said, that was my first thought, but it seemed to hand wavy. Maybe I'm just worrying to much and that's a fine answer. Thank you.
  22. J

    Computer Vision Geometry - Collinear Points In A Pinhole Camera Model

    Homework Statement Prove that a line in 3D space is imaged to a line on the image plane in a pinhole camera model. Homework Equations A 3D point give by (X,Y,Z) will be imaged on the image plane at x = f(\frac{X}{Z}) and y = f(\frac{Y}{Z}) where f is the focal point. The Attempt at...
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