MadSci Network: Physics

Re: How does altitude affect gravity?

Date: Fri Feb 12 08:07:03 1999
Posted By: Eder Molina, Researcher PhD, Dept. of Geophysics, Institute of Astronomy and Geophysics - USP
Area of science: Physics
ID: 918498165.Ph

As Newton's law stated, if you are closer the attractive mass,
the gravity force (and acceleration) is bigger; if you are
distant from the mass, the force is small.

This principle applies to the Earth's gravity field: if you consider
all the earth's mass concentrated in its center, when you are
in a mountain the gravity force would be smaller than the force
when you are at sea level. Of course the mass of the mountain
also contributes in this balance.

But, in general, we have a mean decrease of 0.3086 x 10-3 cm/s2 per
meter of altitude (this is what we call normal gravity gradient),
i.e., if you stand in a place that is 1 m higher than the mean
sea level, the gravity acceleration will decrease 0.3086 x 10-3 cm/s2
(sorry about the superscript/subscript, but I think you can understand).

The gravity changes in different places in the world because different
places have different mass distribution in subsurface and different
heights above sea level. The combination of these two factors generates
the change in the gravity value (gravity here meaning gravity
acceleration). The centrifugal force also adds a component in this

In fact, in geophysics, gravity is a fundamental method of investigating
the Earth. Measuring the gravity acceleration in several places you 
can determine how the mass is distributed within the Earth.

If you have interest in details about this, please feel free
to contact me.

Best regards

Eder C. Molina
Dept. of Geophysics
Institute of Astronomy and Geophysics
University of Sao Paulo - BRAZIL

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