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Chapter 20
Section 3 The Earth Takes Shape
Objectives
• Describe the formation of the solid Earth.
• Describe the structure of the Earth.
• Explain the development of Earth’s atmosphere
and the influence of early life on the atmosphere.
• Describe how the Earth’s oceans and continents
formed.
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Chapter 20
Section 3 The Earth Takes Shape
Formation of the Solid Earth
• The Earth is mostly made of rock. Nearly three-fourths
of its surface is covered with water.
• Our planet is surrounded by a protective atmosphere
of mostly nitrogen and oxygen, and smaller amounts of
other gases.
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Chapter 20
Section 3 The Earth Takes Shape
Formation of the Solid Earth, continued
• The Earth formed as planetesimals in the solar
system collided and combined.
• From what scientists can tell, the Earth formed
within the first 10 million years of the collapse of the
solar nebula.
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Chapter 20
Section 3 The Earth Takes Shape
Formation of the Solid Earth, continued
• The Effects of Gravity When a young planet is still
small, it can have an irregular shape. As the planet
gains more matter, the force of gravity increases.
• When a rocky planet, such as Earth, reaches a
diameter of about 350 km, the force of gravity
becomes greater than the strength of the rock.
• As the Earth grew to this size, the rock at its center
was crushed by gravity and the planet started to
become round.
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Chapter 20
Section 3 The Earth Takes Shape
Formation of the Solid Earth, continued
• The Effects of Heat As the Earth was changing
shape, it was also heating up. As planetesimals
continued to collide with the Earth, the energy of
their motion heated the planet.
• Radioactive material, which was present in the
Earth as it formed, also heated the young planet.
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Chapter 20
Section 3 The Earth Takes Shape
Formation of the Solid Earth, continued
• After Earth reached a certain size, the temperature
rose faster than the interior could cool, and the rocky
material inside began to melt.
• Today, the Earth is still cooling from the energy that
was generated when it formed.
• Volcanoes, earthquakes, and hot springs are effects
of this energy trapped inside the Earth.
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Chapter 20
Section 4 Planetary Motion
Objectives
• Explain the difference between rotation and
revolution.
•Describe three laws of planetary motion.
• Describe how distance and mass affect
gravitational attraction.
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Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy
• Each planet spins on its axis. The spinning of a
body, such a planet, on its axis is called rotation.
• The orbit is the path that a body follows as it
travels around another body in space.
• A revolution is one complete trip along an orbit.
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Chapter 20
Section 4 Planetary Motion
Earth’s Rotation and Revolution
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Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy, continued
• Johannes Kepler made careful observations of
the planets that led to important discoveries about
planetary motion.
• Kepler’s First Law of Motion Kepler
discovered that the planets move around the sun
in elliptical orbits.
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Chapter 20
Section 4 Planetary Motion
Ellipse
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Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy, continued
• Kepler’s Second Law of Motion Kepler noted that
the planets seemed to move faster when they are
close to the sun and slower when they are farther
away.
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Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy, continued
• Kepler’s Third Law of Motion Kepler observed
that planets more distant from the sun, such as
Saturn, take longer to orbit the sun.
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Chapter 20
Section 4 Planetary Motion
Newton to the Rescue!
• Kepler did not understand what causes the plans
farther from the sun to move slower than the closer
planets.
• Sir Isaac Newton’s description of gravity provides
an answer.
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Chapter 20
Section 4 Planetary Motion
Newton to the Rescue! continued
• The Law of Universal Gravitation Newton’s law
of universal gravitation states that the force of gravity
depends on the product of the masses of the objects
divided by the square of the distance between the
objects.
• According to this law, if two objects are moved
farther apart, there will be less gravitational attraction
between them.
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Chapter 20
Section 4 Planetary Motion
Newton to the Rescue! continued
• Orbits Falling Down and Around Inertia is an
object’s resistance to change in speed or direction
until an outside force acts on the object.
• Gravitational attraction keeps the planets in their
orbits. Inertia keeps the planets moving along their
orbits.
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Chapter 20
Section 4 Planetary Motion
Gravity and the Motion of the Moon
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Chapter 20
Standardized Test Preparation
Interpreting Graphics
The diagram below models the moon’s orbit around the
Earth. Use the diagram below to answer the questions
that follow.
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Chapter 20
Standardized Test Preparation
1. Which statement best describes the diagram?
A Orbits are straight lines.
B The force of gravity does
not affect orbits.
C Orbits result from a
combination of gravitational
attraction and inertia.
D The moon moves in three
different directions depending
on its speed.
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Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 20
Standardized Test Preparation
1. Which statement best describes the diagram?
A Orbits are straight lines.
B The force of gravity does
not affect orbits.
C Orbits result from a
combination of gravitational
attraction and inertia.
D The moon moves in three
different directions depending
on its speed.
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Chapter 20
Standardized Test Preparation
2. In which direction does gravity pull the moon?
F toward the Earth
G around the Earth
H away from the Earth
I toward and away from
the Earth
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Chapter 20
Standardized Test Preparation
2. In which direction does gravity pull the moon?
F toward the Earth
G around the Earth
H away from the Earth
I toward and away from
the Earth
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Chapter 20
Standardized Test Preparation
3. If the moon stopped moving, what would happen?
A It would fly off into space.
B It would continue to orbit
the Earth.
C It would stay where it is in
space.
D It would move toward the
Earth.
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Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 20
Standardized Test Preparation
3. If the moon stopped moving, what would happen?
A It would fly off into space.
B It would continue to orbit
the Earth.
C It would stay where it is in
space.
D It would move toward the
Earth.
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Copyright © by Holt, Rinehart and Winston. All rights reserved.