# Does gravity change?

by Gliese123
Tags: gravity
 P: 144 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?
 Math Emeritus Sci Advisor Thanks PF Gold P: 39,691 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.
 P: 144 Okay. Maybe.
 P: 63 Does gravity change? Also, an app on a cellphone may not be so exact and precise. It also changes with height but only very little.
 P: 177 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.
 Mentor P: 22,316 Frankly, I think 3% is pretty darn good for a cell phone!
 P: 166 The cell phone itself might be moving and affects the acceleration.
P: 144
 Quote by russ_watters Frankly, I think 3% is pretty darn good for a cell phone!

It's a Samsung Galaxy 2. :)
P: 144
 Quote by dalcde The cell phone itself might be moving and affects the acceleration.
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 $\mu$s. (Power 0.25 mA).
 P: 144 And the Moon doesn't cause more than +- 2 µm/s² ..
P: 111
 Quote by Gliese123 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 $\mu$s. (Power 0.25 mA).
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.
Mentor
P: 15,205
 Quote by Gliese123 It says that it has a resulution of 0.0047884034 m/s2.
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.
 P: 1 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. :)
 Mentor P: 15,205 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.
 P: 11 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.
 P: 126 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
Mentor
P: 15,205
 Quote by alexg 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
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.
 P: 1 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?

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