Accelaration due to gravity

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Accelaration due to gravity

Postby soumaz07 » Fri Dec 21, 2007 3:29 am

Does the accelaration due to gravity change during different phases of moon.If it changes then by how much? :D
PLEASE SEND ME THE ANSWER WITHIN 5 DAYS.
THANK YOU!!
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Re: Accelaration due to gravity

Postby barretttomlinson » Mon Dec 24, 2007 11:52 am

Thank you for asking a very profound question!!

If you think about it the answer about whether the moon influences the apparent force of gravity has to be yes because the ocean has tides of about 0.5 meters that follow the moon. Answering the question of how much gravity is affected is much harder. The first question is how do you measure how strong gravity is? Then, how accurately can you measure gravity? And finally, is gravity constant over time? It was hard to find those answers, but the best I could do is:

accepted value for gravity is: ABOUT 981,000,000 microgals
approximate variation due to sun and moon is between 30 and 300 microgals, two thirds due to moon, on third due to sun
current instruments measure gravity changes to about 1 micro gal

gal stands for galileo; 1 galileo = 1 cm/sec^2; 1,000,000 microgal = 1 gal

see: http://answers.google.com/answers/threadview?id=94122

Here is a paper describing how one group of scientists measured the force of gravity recently. It contains a figure showing gravity measurements over 18 hours, showing a variation of about 100 microgal due to sun and moon influences.

see: http://www.ngs.noaa.gov/PUBS_LIB/BigG/bigg.html

Here are 2 websites with lots of interesting information on measuring gravity, and how you can use that information:

http://www.ngs.noaa.gov/GRD
http://www.ngs.noaa.gov/GEOID

It is also very interesting to research the various ways people measure gravity.

This sounds like a really fantastic topic for a science fair project, though it may be most suitable for an advanced high school student.

Again, thank you for asking a really, really good question. I had great fun researching the answer over this weekend.
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Re: Accelaration due to gravity

Postby barretttomlinson » Mon Dec 24, 2007 7:14 pm

The first link on the previous post is bad - the correct link is:

http://answers.google.com/answers/threadview?id=194122

The links on this page include a link to an analysis of how to compute the effect on the earth by the moon, discussion of sensitivity of sensors that might be used in a gravity measuring instrument, and other useful stuff.

I regret the error.
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Re: Accelaration due to gravity

Postby soumaz07 » Tue Dec 25, 2007 3:56 am

Thank You ,for the reply.
Please send me a more precise answer.If there is a change in the accelaration due to gravity due to different phases of moon,then please try to mail me the changes . :D
PLEASE SEND ME THE ANSWER WITHIN 5 DAYS.
THANK YOU!!
soumaz07
 
Posts: 6
Joined: Fri Dec 21, 2007 1:48 am

Re: Accelaration due to gravity

Postby barretttomlinson » Tue Dec 25, 2007 10:46 am

Here is the best data I have seen. New moon =Feb 12,2002, Full Moon =Feb 27, 2002, First Quarter =Feb 20, 2002, Last Quarter =Feb 4, 2002 & March 4, 2002.

http://www.ngs.noaa.gov/GRD/SG/sg_data.html

Does this fully answer your question? If not, please clarify what you need to know.
I have seen a reference to the calculations to correct gravity observations for the effects of sun and moon. It was said the calculations are exceptionally tedious to do by hand, but there are computer programs to do them.

Good luck on your project!
Last edited by barretttomlinson on Tue Dec 25, 2007 6:33 pm, edited 1 time in total.
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Re: Acceleration due to gravity

Postby davidkallman » Tue Dec 25, 2007 11:27 am

soumaz07 wrote:Please send me a more precise answer.If there is a change in the accelaration due to gravity due to different phases of moon,then please try to mail me the changes .


Hi soumaz07,

We're just getting used to the new sciencebuddies format.

As I understand it, you're in grades 6-8.

Is the nature of your post that you need:

1. references appropriate to your grade level or
2. help going through the references?
Cheers!

Dave
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Re: Accelaration due to gravity

Postby sciencebuddy » Wed Mar 04, 2009 6:14 pm

Newton's Law of Universal Gravitation is explained in this link
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html

i'm not 100% certain, but you can try to determine the acceleration due to the mass and distance of the moon from earth and see how that compares to the gravity (9.8m/s^2) due to the sun.

obviously, the most well known example of how the moon affects gravitational force is the tides. this will probably be the best way of doing a project related to gravitational force.

-Dan
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Re: Accelaration due to gravity

Postby Craig_Bridge » Thu Mar 05, 2009 3:03 pm

Waxing and waning philosophically, one can argue that the gravitational acceleration constant caused by the mass of the earth does not change if the mass of the earth does not change.

What does change very slightly is the direction of the gravitational force vector caused by relocation of mass on the earth caused by external factors (moon and sun location). The daily rotation of the earth causes most of the tidal changes because of the moon's gravitational force which is primarily responsible for the slight changes in the earth's center of gravity.

These slight changes are only secondarily related to the phases of the moon which change when the high and low tide occurs for any given spot on the earth and this only occurs because the moon's orbit around the earth is slightly eliptical (not quite circular). Unfortunately, the apogee and perigee of the moon's orbit is NOT a constant relation to the phase of the moon because the sun placement changes through 360 degrees in about 365.25 days. The sun-moon-earth angle is responsible for how much of the moon surface someone on the earth sees as lighted by the sun. The apogee and perigee is what affects the lowest low tide and highest high tide respectively.

Also note that there is a gravitational acceleration constant for the moon and sun and these resulting gravitational forces will also be acting on anything in the vicinity of the earth. All the gravitational forces on an object add using vector mathmatics so the net apparant gravitational force WILL change and thus the net effective gravitational acceleration will change.

But this is way too deep into math and philosophy for what would normally be considered your grade level material. I'm not saying you can't understand it, but it is going to take some reading and study beyond what is normally your grade level.

Your simple questions were not sufficiently defined to prevent a deep philosophical discussion for which there is no one answer. It comes down to what you specifically mean with respect to gravitational acceleration - was it "net" or combined from all sources or was it specifically the contribution from the earth. If you want precise answers, you have to form precise questions.
-Craig
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