Value for gravity?

1. Nov 5, 2003

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!

2. Nov 5, 2003

chroot

Staff Emeritus
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

3. Nov 5, 2003

Ambitwistor

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?)

4. Nov 5, 2003

Tawcan

Oops you guys are right.

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