Gravity Near Earth
Surface Gravity

We think of gravitational acceleration as g = 9.8 m/s2.

This value is due to the universal gravitational force.
• Earth’s radius: 6.37 x 106 m
• Earth’s mass: 5.97 x 1024 kg

Calculate: g = F/m = GM/r2 = 9.81 m/s2.
Variations

Gravity varies over the surface of the Earth.
•
•
•
•

The height of the surface varies – so the radius does, too
The material under the surface is not uniform
The earth is spinning
The tides affect the earth as well as the oceans
The unit of gravitational acceleration used on the
Earth is the Galileo.
• 1 Gal = 1 cm/s2
• g = 981 Gal
Effect of Height

High areas have a greater
distance from the center of
the Earth.

An increase of 1 km should
decrease g by 300 mGal.
r2
2h
ag

g
(
1

)
2
( r  h)
r
USGS
Earth’s Rotation

The earth is made of layers
of different types of rock.

These rocks can move due
to daily tides.
• Much less than ocean tides
• Period is 12 hours like the
ocean
Colorado
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Latitude Changes

As the Earth spins the
equator slightly bulges.

The radius is about 22 km
bigger compared to the pole.

Expect a few Gal difference
from equator to pole.
• Equator: 9.780 m/s2
• North Pole: 9.832 m/s2
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Centripetal Correction

The spin of the earth creates
a centripetal acceleration
toward the axis of the earth.
a = w2r

The effect is like accelerating
down in an elevator.

Items weigh less at the
equator.
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