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Science

Measuring Gravity in Your Basement 37

Posted by michael from the 9.8-m/s^2 dept.
Jack Durian writes "John Walker, the founder of Autodesk/co-author of AutoCAD has some fun playing pretend experimentalist, measuring gravity in his basement."
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Measuring Gravity in Your Basement More

Measuring Gravity in Your Basement

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  • "I would go to the building superintendent and
    offer him a brand-new barometer if he will tell me
    the value of G"
  • The article doesn't mention (iirc) how you account for an minimize the effect of electromagentic forces. Is it the material you choose. I suppose if you juxtapose the spheres and the attraction is the same you have accounted for it (because of polarity).

  • Unpatriotic (Score:3, Funny)

    by kippy ( 416183 ) on Tuesday April 16, 2002 @10:47AM (#3350172)
    How could we let a traitor like John Walker tell us how to measure gravity.

    It's unamerican!

  • Other fun experiments to try (Score:3, Interesting)

    by QuantumG ( 50515 ) <qg@biodome.org> on Tuesday April 16, 2002 @11:23AM (#3350485) Homepage Journal
    Put a ring of optic fibre on one of the arms and pump laser light into it. Measure how long it takes for the arm to respond. Lesson: Mass and Energy bends space time.

  • How about... (Score:2, Funny)

    by jo42 ( 227475 )
    John Walker come up with a device to measure the Density of Larry Elison?
  • ...did he make absolutely certain that the whole apperatus was electrostatically neutral?


    If not, then as pointed out on the web page, the relative strength of the electrostatic force (which is also inverse square) is far larger than the puny force of gravity and it could swamp the measurement.


    For your information, a common way to measure G (the gravitational constant) in the lab is by using an oscillating torsion balance and detecting the frequency change due to the introduction of large masses in the vicinity.


    What the heck is a dyne anyhow, what's wrong with good old' SI? NIST [nist.gov] for those interested there's a converter between all the old interesting things like the pole, perch, hogshead, American mile, British mile, American short ton, British long ton and various other devients of the mind Here [ex.ac.uk]

    • Re:Nice experiment - but .... (Score:1, Informative)

      by Anonymous Coward
      For your information, it's metric (cgs).
    • The reason he used dynes and stuff (part of the cgs - centemeter, gram, second - system) and not the SI (formerly the mks - meters, kilograms, seconds) system is that most of his measurements would have been different by many orders of magnitude. Who wants to see something with a force of 0.0000001 newtons (kg*m/sec/sec) when you could see 0.01 dynes (gm*cm/sec/sec)?

  • 6.67259 (Score:3, Interesting)

    by CyberDruid ( 201684 ) on Tuesday April 16, 2002 @12:26PM (#3351071) Homepage
    The gravitational constant is an elusive thing. When I studied quantum physics, the lecturer, who was one of the members of the Nobel Prize committee (I live in Sweden), told us that one of the "sure" ways (there are a few grand questions in physics which has this status, others probably include: finding the mass of neutrinos, evidence for the Higg's Boson, the decay time for protons, etc) to get a Nobel Prize is to measure G as exactly as the other physical constants are known (i.e roughly to the same number of significant digits).

    • Re:6.67259 (Score:1, Informative)

      by Anonymous Coward
      Check out "Gravity" by Chuji Tsuboi.

      He describes the International Geodetic or Gravimetric Network or something, they have gravimeters about the surface of the Earth or measured readings to get an idea of the varying gals of gravity. A gal is a unit of gravity. Over the surface of the planet it is something like around 980 gals, varying by tens to more of milligals. Normally people think the acceleration of gravity is 9.8 m/s^2, or, -9.8 m/s^2.

      http://www.mines.edu/fs_home/tboyd/GP311/MODULES /G RAV/NOTES/gravunits.html

      The Earth is a spheroid oblate. The southern hemisphere is negligably, barely, thicker through the latitudes. Maybe the center of gravity of the galaxy is to the south, or, maybe just this part of the galaxy.
      • Everybody knows that... However g is not the same as G. G is the universal constant that governs all gravity and that is what we need to measure. There is nothing fundamental about g.
  • Uh, I'm not a lawyer, but I think his apparatus may violate this patent [slashdot.org].

    • No, see... his swing is twisting, not swinging sideways. :hits head: Hey! Maybe I can patent twisting back and forth on a swing! :runs to patent office: (If you can't tell, this post is oozing with sarcasm)

  • The Obvious (Score:1, Insightful)

    by airship ( 242862 )
    What blows me away isn't the fact that he demonstrated gravitational force in his basement, it's the second demonstration that showed how Archimedes (or any other Greek natural philosopher) could have done the very same thing with sticks, string, and stones. Incredible! I've always been very, very intersted in history and raw information (or, generally, the lack of it). It's pretty well been shown that the Egyptians could have created a pretty powerful computer using fluidics bricks, and that the Chinese could have build hang gliders using silk and bamboo. Now it turns out we could have had Newton 1900 years earlier. Amazing how much difference just a little bit of knowledge can make.

  • Sensitive (Score:3, Interesting)

    by vossman77 ( 300689 ) on Tuesday April 16, 2002 @04:35PM (#3353468) Homepage
    I had to do this experiment for an undergraduate physics lab.

    I have trouble believing this setup will work, because
    • The experiment is open air and very sensitive to any movement in air flow and also sound. When closing the door to the room, you can consider you're experiment invalid.

    • A camera is nice for measuring position, but it can also be measured directly by passing the metal balls through capacitor like setup creating much more accurate direct measurements.

    • Typical measurements yield numbers off by a factor of two. Making it very hard to get a good result. The current constant was measure very far underground where the mass of the Earth is more accurate. A laser has been typically used in this case, but doesn't use a computer.

    Experiments today are done with a Cavendish apparatus, which very similar to the one shown. Here's a link [washington.edu] with some pics.

    This is unreserached thought, so don't come down on me too hard. I am just recalling from my youth

    vossman

    • Re:Sensitive (Score:3, Interesting)

      by bcrowell ( 177657 )
      I had some students do experiments using Walker's technique. Some photos are in chapter 10 of this [lightandmatter.com] online book. You're right that it's very very touchy, and you're not likely to get more than an order-of-magnitude estimate of G (although an o.o.m. estimate is still interesting). Air currents are a big problem if you're in the room, but once you leave the room, it's not an issue unless the heating vents are open or something. That's the point of using a video camera -- so you can be out of the room. One thing that worries me is that entering the room to insert the masses may cause air currents big enough to disturb the apparatus. We never got good quantitative results.

      Walker is an interesting character. He has some very cool free-as-in-beer server-side astronomy software on his site, although it's a shame it doesn't seem to be open source. But then, this is a guy who helped found Autodesk, which used hardware dongles to prevent copying...

      • There is a much easier way to get an order of magntitude estimate of G: you know g (accererlation of gravity on Earth), Earth's radius and you can estimate the mass of Earth to (as it turns out) within a factor of 2 by knowing rock/metal typically has a density of about 3-4 g/cm^3. (You can do better if you understand how rocks and metals compress under pressures.) From that, you get G to within about a factor of 2.

        Of course, that isn't nearly as fun as doing the experiment!
  • Man, this Walker guy sure keeps busy, he's a Texas Ranger, a Taliban, he's a scientist...wow!
  • I was interested in how accurately someone could measure G without specialist equipment.

    From the article:

    We, however, aren't going to measure anything--we're only interested in observing universal gravitation.
    Most misleading headline ... ever?

  • ...unless you don't care about accurate results.

    If you're looking for accuracy, you'll have to perform the experiment at least a hundred miles from any type of human civilization, including any streets with cars driving by. Also make sure no planes fly by anywhere in the area, and that there's no wind outside to create vibrations of any sort.


  • You want to see what Gabe Luther and and William Towler,current "holders" of Big G, used to measure it ? Here's a great shot of the torsion balance device [washington.edu] from this short summary [washington.edu] .

    Here's a link to the press room [lanl.gov] at LANL Look for "17) Measuring the Gravitational Constant ("Big G") -- In the Lab of Gabe Luther, Los Alamos scientist. Sound bite on methodology." - no link but an interesting page of resources.

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