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Does gravity change?

  1. Aug 12, 2011 #1
    Hi all! I didn't know where to put this topic but since I thought it's general for all planets and gravitational aspects I put it here. I've made some gravity measurements, some quite simple with a accelerometer on my cellphone with different apps, but also with a good accelerometer several times and got different results day to day at the same spot. I live in southeast Sweden near the coast. I usually get 10.1 m/s2, which is quite "high", but sometimes when I measure I get ~9.8 m/s2 and I'm very exact when I do it. Even though the differences is little notable, I wonder: Is the gravity actually changing, day to day?
     
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  3. Aug 12, 2011 #2

    HallsofIvy

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    I suspect you are just running into measurement errror. The acceleation due to gravity at one point on the earths surface should not change "day to day". The acceleration due to gravity at different points on the earth can vary due slightly to things like altitude and the presence of unusually dense minerals but I doubt you would get something as high as 10.1.
     
  4. Aug 12, 2011 #3
    Okay. Maybe.
     
  5. Aug 12, 2011 #4
    Also, an app on a cellphone may not be so exact and precise. It also changes with height but only very little.
     
  6. Aug 12, 2011 #5
    Unless you take a lot of measurements with the same device (in the same location), it may be difficult to determine the source of the differences in measurements (and even then one would need to know how the devices work to determine sources of systematic bias). If the devices only give you three significant figures, the difference between the two measurements is probably not significant.
     
  7. Aug 12, 2011 #6

    russ_watters

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    Frankly, I think 3% is pretty darn good for a cell phone!
     
  8. Aug 12, 2011 #7
    The cell phone itself might be moving and affects the acceleration.
     
  9. Aug 14, 2011 #8

    It's a Samsung Galaxy 2. :)
     
  10. Aug 14, 2011 #9
    Well, I had it still on the ground without touching it. And the tilt was 0 in all directions, x, y & z. And let say that even though there are an small error in the measurment sensors, and it's laying straight on the ground without anything or anyone is making it move, then why is the gravity changing after a couple of hours when I come back? I guess that even though that it showing different results, then the error marginal doesn't really matter. It says that it has a resulution of 0.0047884034 m/s2. (Which is really good for a cellphone). And a maximum range of 19.6133 m/s2. Minimum delay: 15000 [itex]\mu[/itex]s. (Power 0.25 mA).
     
  11. Aug 14, 2011 #10
    And the Moon doesn't cause more than +- 2 µm/s² ..
     
  12. Aug 15, 2011 #11
    If you believe that it can accurately and precisely measure gravity to 1 part in 2000 with that phone, I have a special deal on some swamp land for you.

    The person who listed 8 significant digits for resolution doesn't know jack.
     
  13. Aug 16, 2011 #12

    D H

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    Resolution and accuracy are two very different things.

    Note that 1 g = 9.80665 m/s2 by definition, and that 9.80665 / 2048 = 0.0047884034. All that that 0.0047884034 m/s2 figure means is that the MEMS accelerometer in your iPhone is internally sensing acceleration in terms of counts, with a count of 1 supposedly being 1/2048 g, a count of 2 supposedly being 2/2048 g, a count of 2048 being 1 g, etc. The resolution of your iPhone accelerometer is 1/2048 g. There are many reasons why a power of two was chosen for the scale factor.

    That scale factor of 1/2048 that converts counts to acceleration is a hard coded value. Whether a count of 2048 is truly 1 g is a very different matter. The correct scale factor depends on the accelerometer itself and is not constant. The accuracy of your iPhone accelerometer is nowhere near 1/2048 g. That accelerometer is a cheap, cheap, cheap MEMS accelerometer. There's a lot of variation from one accelerometer to another. The correct scale factor also depends on temperature and on the voltage supply; those cheap MEMS accelerometers are somewhat temperature sensitive and are very sensitive to the input voltage. This sensitivity to voltage and temperature (and to other things) are most likely what is giving you different readings on different days.
     
  14. Mar 15, 2012 #13
    Earth is causing gravity(on earth).. and the moon.. and the sun causes a significant pull also. If any of these bodies changes shape or mass, gravity will change. We know earth has quite a lot of liquid and gass that moves around, water, magma, air. So yes gravity changes.. everytime any object moves infact.. I don't know how significant theese changes are.. but still I can say 100% definatly that gravity changes from day to day. However I doubt you can use this information for the purpose of jumping higher etc. when conditions are favorable.. the moon and sun at the right place and magma density below your feet is low. But feel free to try. :)
     
  15. Mar 15, 2012 #14

    D H

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    Variations in gravity due to the Moon and Sun together amount to 0.2 micro g or so (1 micro g is one one millionth of Earth standard gravity). You would need something a lot more precise and a lot more accurate than the cheap (a few dollars?) MEMS accelerometer on a cellphone to measure something these tiny tidal accelerations.
     
  16. Mar 15, 2012 #15
    I've worked with MEMS accelerometers in the past. There are multiple sources of error.

    Firstly the accelerometer on the phone is not that accurate. I suspect it is using a 6D gyroscope/accelerometer chip. These are manufactured with size and cost in mind, not accuracy.

    Secondly, most phone OSes have inbuilt sensor filters and fusion built in. So what you are reading may not the pure raw data from the accelerometers.
     
  17. Mar 15, 2012 #16
    However, gravity on earth does vary, due to density differences.

    The larger lumps and the red areas are areas of greatest mass.

    http://discovermagazine.com/2007/mar/grace-in-space

    http://discovermagazine.com/2007/mar/grace-in-space/euro-map-700.jpg [Broken]
     
    Last edited by a moderator: May 5, 2017
  18. Mar 16, 2012 #17

    D H

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    That is a map of gravity anomalies, and not gravitational acceleration. The several thousands of milligals variations due to latitude and several hundreds of milligals variations due to altitude have been removed from this picture.
     
    Last edited by a moderator: May 5, 2017
  19. Nov 9, 2012 #18
    If gravity or gravitational energy isolated at a single point, Would that change everything as we know it? Could there be a rate of change of acceleration? How many infinite door does that open?
     
  20. Feb 2, 2013 #19
    This is interesting, and how i found this thread.

    At home, my bathroom door acts differently, i mean, you know when you just push the door until its almost closed, you know, without pushing until the knob locks or you have to push down the handle to open it again.
    One it will stay there without any signs of wanting to open again, but the next day, the door opens wide open and pretty fast no matter how firmly i push it in.
    I may have explained this poorly, but does anybody know what i mean, and can explain that?
    Its like the angle of the ground/house is changing from day to day :uhh:
     
  21. Feb 2, 2013 #20

    Drakkith

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    There are a dozen possibilities but none of them have anything to do with gravity changing. Expansion due to temperature changes, humidity, and other things are probably the culprit.
     
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