Download File - Mr. Catt`s Class

6-2 thru 6-3
The EarthMoon
System
© 2007 Jones and Bartlett Publishers
Courtesy of NASA/JPL/Northwestern University
Chapter 6
6-2 The Tides
1. The Moon exerts a gravitational force on each
individual part of the Earth. This tidal force
varies in strength and direction over the Earth
causing it to deform.
2. A unit mass on the side of the Earth closest to
the Moon feels a gravitational force from the
Moon about 3% greater than the force on a unit
mass at the Earth’s center, which in turn is 3%
greater than the force on a unit mass at the far
side of the Earth.
3. On the side of the Earth nearest the Moon, water
feels a greater force and flows to the area under
the Moon, causing a high tide.
Figure 6.10b: In the Earth-Moon system, the
gravitational forces result into a motion around the
common center of mass and the differences in the
forces result into tides.
© Andrew J. Martinez/Photo Researchers, Inc.
© Andrew J. Martinez/Photo Researchers, Inc.
Figure 6.09b: High tide
Figure 6.09a: Low tide
4. A high tide on the opposite side of the Earth
(farthest from the Moon) occurs because the center
of the Earth feels a greater force toward the Moon
than water on that side, so the main body of the
Earth is pulled away from the water, resulting in
another high tide.
5. Differential gravitational pull on the various parts of
the Earth results in two areas of the Earth
experiencing high tides.
On most days on the Earth there are two high tides and two
low tides.
6. As the Earth rotates on its axis, the Moon revolves
around the Earth.
Because the Moon is not stationary, the Earth must turn for an
additional 50 minutes each day before a spot on the Earth
returns to the same position with respect to the Moon.
This is what causes the high tides and the rising and setting of
the Moon to occur about 50 minutes later each day.
Figure 6.11
8. A spring tide is the greatest difference between
high and low tide, occurring about twice a month
when the lunar and solar tides correspond.
9. A neap tide is the least difference between high and
low tide, occurring when the solar tide partly
cancels the lunar tide (i.e., when the solar tides are
90° from the Moon’s).
Figure 6.12a: During new or full moon,
the tidal forces from the Moon and Sun
add up to give us enhanced (spring)
tides.
Figure 6.12b: During first and third
quarter Moon, the two tidal forces
cancel partially, giving us smaller
(neap) tides.
Question 1 (6-2 thru 6-3 PPT Questions)
The Earth has a high tide toward the Moon and away
from the Moon. What is the reason for this?
Rotation and Revolution of the Moon
1. The period of the Moon’s rotation exactly matches its
period of revolution. This is caused by tidal forces,
and as a result the Moon keeps the same face toward
Earth at all times.
2. There are frictional forces between the solid Earth
and its oceans. The Earth’s motion tends to drag the
tides along with it, so that a high tide is not directly
under the Moon but is farther to the east.
3. Tidal friction (i.e., friction forces that result from
tides on a rotating object) has slowed Earth’s
rotation over time.
4. As a result of tidal interactions, the Moon is pushed
farther away from Earth.
Precession of the Earth
1. Precession is the conical shifting of the axis of a
rotating object, also known as wobbling.
2. The Earth is not a perfect sphere; its equatorial
diameter is about 26 miles greater than its polar
diameter. Earth’s spinning on its axis causes it to
flatten slightly at the poles.
3. Oblateness is a measure of the “flatness” of a
planet, calculated by dividing the difference
between the largest and smallest diameter by the
largest diameter.
4. The Earth precesses very slowly, requiring a period
of about 26,000 years.
5. As the Earth precesses, stars different from Polaris
(or no visible stars) occupy the position near the
Earth’s north celestial pole.
A corresponding effect is that the position of the
vernal equinox changes over the centuries.
Question 2 (6-2 thru 6-3 PPT Questions)
Describe spring and neap tides. What are their
differences and what causes them to occur?
6-3 Earth
The Interior of the Earth
1. Density is the ratio of an object’s mass to its
volume. Earth’s average density is 5.52 g/cm3.
(The density of water is 1 g/cm3; of aluminum
2.7 g/cm3; of iron 7.8 g/cm3.)
2. Earth’s interior is made up of three layers.
(a) Crust is the thin (<100 km) outermost layer of the Earth; it
has a density of 2.5–3 g/cm3.
(b) Mantle is the thick (2900 km) solid layer between the crust
and the Earth’s core; it has a density of 3–9 g/cm3. The
crust “floats” on top of the mantle.
(c) Core is the central part of the Earth, composed of a solid
inner core and a liquid outer core. The core is probably
composed of iron and nickel and its density ranges from
9–13 g/cm3.
Figure 6.16: The interior of the Earth, showing its primary layers.
3. This pattern of increasing density is called
differentiation and is caused by the sinking of
denser materials toward the center of planets or
other objects.
4. We know about the makeup of the Earth’s interior
by analyzing travel times of two types of waves
generated by earthquakes:
the P-waves (primary waves, analogous to waves produced
by pushing a spring back and forth), and
the S-waves (secondary waves, analogous to the waves
produced by shaking a rope attached to a wall up and
down).
Plate Tectonics
1. Alfred Wegener is credited with first developing the
idea of continental drift—the gradual motion of the
continents relative to one another.
2. Rift zone is a place where tectonic plates are being
pushed apart, normally by molten material being
forced up out of the mantle.
Figure 6.17 The arrangement of Earth's
continents about 200 million years ago
Figure 6.17: The arrangement of Earth's
continents today
3. The theory of plate tectonics states that sections of
the Earth’s crust move across the underlying
mantle. There are about 12 tectonic plates that
extend about 50–100 km deep.
4. Over millions of years, moving plates—often
crashing into one another—have caused the
continents to “drift,” mountains to be uplifted,
ocean trenches to form, and earthquakes to be
unleashed.
Figure 6.18: The major tectonic plates of the Earth
Earth’s Atmosphere
1. Earth’s atmosphere consists of
about
• 78% nitrogen (N2),
• 21% oxygen (O2),
• with minor amounts of water vapor
(H2O), carbon dioxide (CO2), argon (Ar),
and trace amounts of ozone (O3).
2. Troposphere is the lowest level of
the Earth’s atmosphere
• it contains 75% of the atmospheric mass
• it is about 11 km (7 mi) deep
•
and is where weather occurs.
Figure 6.20: The temperature of the atmosphere varies with altitude.
3. The troposphere receives most of its heat from
infrared radiation emitted from the ground; thus, the
temperature of the troposphere decreases as one
goes higher.
4. About 50 km above the Earth’s surface is the ozone
layer.
• Ozone is an efficient absorber of the Sun’s UV radiation.
• This absorption causes the temperature of the Earth’s
atmosphere to peak at the ozone layer.
5. The ozone layer has protected life on Earth for
billions of years.
The release of chlorofluorocarbons during the 20th century
has reduced, through molecular interactions, the amount
of ozone available to protect us.
This is an issue of international concern and underscores the
difficulties encountered when competing interests—
political, economic, scientific—collide.
Earth’s Magnetic Field
1. A magnetic field exists in a region of space if
magnetic forces can be detected there.
2. The magnetic poles of the Earth are not located at
its poles of rotation. The location of the magnetic
poles changes with time.
3. According to the dynamo model, the Earth’s (and
other planets’) magnetic field is due to currents
within a molten iron core.
4. The three main conditions for generating a magnetic
field are: (i) a seed magnetic field, (ii) a conducting
fluid, and (iii) an energy source to move the fluid in
an appropriate pattern.
5. The Van Allen belts are doughnut-shaped regions
composed of charged particles (protons and
electrons) emitted by the Sun and captured by the
magnetic field of the Earth.
6. Auroras are caused by charged particles trapped in
the Earth’s magnetic field striking atoms and
molecules in the upper atmosphere.
Figure 6.24a: The Van Allen belts are regions where the magnetic field of
the Earth traps charged particles from the Sun.
Question 3 (6-2 thru 6-3 PPT Questions)
Craters have been formed on the Earth and on the
Moon by meteorite impact. The Earth has a much
stronger gravitational field than does the Moon, and
yet we find more craters on the Moon. Explain this
apparent contradiction.