# Value for gravity?

#### Tawcan

Hi, new to here...

I'm looking for the formula to determine accureate value of g when value of longitude and height above sea level are given.

If you know if would be helpful if you could post a site that has the formula. (Want to have it as a reference).

Thanx!

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#### chroot

Staff Emeritus
Gold Member
I assume you mean latitude, not longitude. g varies with latitude because of the earth's rotation.

Here's one page that may help:

http://www.npl.co.uk/pressure/faqs/altgrav.html

In general, you know Newton's law of universal gravitation:

F = G M m / r^2

and you know Newton's second law of motion:

F = m a

so you can substitute and solve for a:

a (g) = G M / r^2

Plug in the earth's mass for M and your distance from its center as r. Voila. This calculation, of course, does not include the small effect of the rotation.

- Warren

#### Ambitwistor

Originally posted by Tawcan
I'm looking for the formula to determine accureate value of g when value of longitude and height above sea level are given.
Do you want a theoretical prediction, assuming the Earth is a uniform rotating sphere, or do you want some kind of empirically-fit formula from measured data, taking into account all the irregularities in the Earth?

(By the way, do you mean latitude, not longitude?)

#### Tawcan

Oops you guys are right.

Yep that's exactly the formula I'm looking for! Thanx for your help! :)

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