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Gravity and Density

  1. Oct 30, 2011 #1
    If I had a balloon full of gas, and inside that balloon a solid canister that had a much smaller volume than the balloon but could hold all the gas inside the balloon under pressure, would there be a (detectable or theoretical) change in the gravity generated by forcing all the gas into the canister under pressure?
     
  2. jcsd
  3. Oct 30, 2011 #2
    Yes, due to the potential energy created, but it would be negligable. However, it would change the bouyancy of the gas, due to the reduction in volume, so the net force on the gas would change.
     
  4. Oct 30, 2011 #3
    Would I be able to detect this change with an accelerometer placed immediately above the balloon?
     
  5. Oct 30, 2011 #4

    phinds

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    What is it that the accelerometer would be measuring?
     
  6. Oct 30, 2011 #5
    The acceleration towards a point, in this case it would be both the earth and the compressed gas (due to its heightened gravitational pull (I'm not for a second saying it would be a big gravitational pull, I'm just interested in measuring it))
     
  7. Oct 30, 2011 #6

    phinds

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    If I correctly understand what your saying, then no I don't see how there would be any difference UNLESS the accelerometer moved closer to the center of mass of the gas, since that would cause a tiny increase in the gravity measured by the accelerometer (the amount would not be measurable with anything like today's technology but it would exist)
     
  8. Oct 31, 2011 #7
    Is there any way I can create a change in gravity that I could monitor with DIY lab equipment?
     
  9. Oct 31, 2011 #8

    phinds

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    I don't have any answer for that but I'll bet some of the more clever folks here will.
     
  10. Oct 31, 2011 #9

    Matterwave

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    You have to define what you mean by "change in gravity". If you mean change in the gravitational force felt by some object, then...yes...it's very easy, you just change the mass of that object (cut off a piece of it), the gravitational force will reduce. Take a piece of putty and weigh it, and then cut off a piece and weigh it again, the lower reading shows a reduction in the gravitational force....

    But I suspect this is not what you mean.
     
  11. Oct 31, 2011 #10

    phinds

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    Right ... I was going to suggest that he eat a Big Mac and then reweigh himself, but I came to the same conclusion that you did so bit my tongue.:smile:
     
  12. Oct 31, 2011 #11
    I want to detect a change without touching the object (for example, an accelerometer dangling above an object)
     
  13. Oct 31, 2011 #12

    phinds

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    I don't think you'll be able to do that w/ DIY equipment because the change will be very subtle almost whatEVER you do, relative to the gravity of the earth, which you can't remove from the experiment.

    For example, I believe that the discovery and mapping of mascons in the earth required very sensitive instruments and you are NOT going to come close to producing a mascon.
     
  14. Oct 31, 2011 #13

    Ibix

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    The theory is simple. You hang a mass from a spring and bring another mass up close below it. The spring will stretch slightly and you use Hooke's law to measure the force. Then you try again with a smaller mass and you'll see a smaller stretch.

    The problem is how weak gravity is. Two 1000kg masses with their centers separated by 1m (think about the size of a 1000kg mass) produce around a 0.07mN force. Any spring balance sensitive enough to detect this will snap when you attach a 1000kg mass. If you try a smaller mass then the force goes down (so harder to measure) and (in a double whammy) the system becomes extremely sensitive to thermal noise - draughts, Brownian motion, cars passing in the street, earthquakes thousands of miles away, etc, etc.

    Measuring g between two known masses is the way we calculate G. The pros take years to make a measurement, and they frequently disagree. You could be decades at this and get nothing but noise - sorry.

    Easier: track planetary motions and confirm that the orbits match predictions from theory. More maths and more indirect, but practicable for the dedicated hobbyist, I would think, given that Kepler and Newton have already done the heavy lifting.
     
    Last edited: Oct 31, 2011
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