80
This false-color map shows variations
in the strength of the earth’s gravity.
Purple areas have the strongest
gravity, yellow the weakest. The over-
all trend toward weaker gravity at the
equator and stronger gravity at the
poles has been artificially removed to
allow the weaker local variations to
show up. The map covers only the
oceans because of the technique used
to make it: satellites look for bulges
and depressions in the surface of the
ocean. A very slight bulge will occur
over an undersea mountain, for
instance, because the mountain’s
gravitational attraction pulls water
toward it. The US government
originally began collecting data like
these for military use, to correct for the
deviations in the paths of missiles. The
data have recently been released for
scientific and commercial use (e.g.
searching for sites for off-shore oil
wells).
Chapter 3Acceleration and Free Fall
mass, and the attraction gets weaker with increasing distance. As you ascend
from the seaport of Guayaquil to the nearby top of Mt. Cotopaxi, you are
distancing yourself from the mass of the planet. The dependence on latitude
occurs because we are measuring the acceleration of gravity relative to the
earth’s surface, but the earth’s rotation causes the earth’s surface to fall out
from under you. (We will discuss both gravity and rotation in more detail
later in the course.)
Much more spectacular differences in the strength of gravity can be
observed away from the Earth’s surface:
location
asteroid Vesta (surface)
Earth's moon (surface)
Mars (surface)
Earth (surface)
Jupiter (cloud-tops)
Sun (visible surface)
typical neutron star (surface)
0.3
1.6
3.7
9.8
26
270
10
12
g (m/s
2
)
infinite according to
some theories, on the
order of 10
52
according to others
black hole (center)
A typical neutron star is not so different in size from a large asteroid, but is
orders of magnitude more massive, so the mass of a body definitely corre-
lates with the g it creates. On the other hand, a neutron star has about the
same mass as our Sun, so why is its g billions of times greater. If you had the
misfortune of being on the surface of a neutron star, you’d be within a few
thousand miles of all its mass, whereas on the surface of the Sun, you’d still
be millions of miles from most if its mass.