# 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!

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

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! :)