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Roger A. Freedman • William J. Kaufmann III
Universe
Eighth Edition
CHAPTER 4
Gravitation and the
Waltz of the Planets
Reading Assignment – Chapter 5 (The Nature of
Light)
Homework – Chapter 3 Quiz due Wednesday 9/15
by 9 PM
Epicycles are used in a geocentric model to explain
the
A.
B.
C.
D.
E.
Q4.3
eastward motion of Mars.
westward motion of Mars.
phases of Venus.
appearance of Venus in the early morning or late
evening sky.
phases of the moon.
Epicycles are used in a geocentric model to explain
the
A.
B.
C.
D.
E.
A4.3
eastward motion of Mars.
westward motion of Mars.
phases of Venus.
appearance of Venus in the early morning or late
evening sky.
phases of the moon.
When Jupiter is at opposition (see diagram), it is
overhead (i.e., on the meridian) at
A.
B.
C.
D.
E.
Q4.1
6:00 A.M.
12:00 noon.
6:00 P.M.
midnight.
never at night.
When Jupiter is at opposition (see diagram), it is
overhead (i.e., on the meridian) at
A.
B.
C.
D.
E.
A4.1
6:00 A.M.
12:00 noon.
6:00 P.M.
midnight.
never at night.
A planet in an elliptical orbit around the Sun moves
most rapidly when it is
A.
B.
C.
D.
E.
Q4.6
at aphelion.
at perihelion.
halfway between perihelion and aphelion.
closer to aphelion than perihelion.
None of the above; it has a constant speed.
A planet in an elliptical orbit around the Sun moves
most rapidly when it is
A.
B.
C.
D.
E.
A4.6
at aphelion.
at perihelion.
halfway between perihelion and aphelion.
closer to aphelion than perihelion.
None of the above; it has a constant speed.
Kepler’s three laws
A.
B.
C.
D.
E.
Q4.5
were initially derived from fundamental physics
laws.
were initially found from observation and are
empirical rules that describe how the planets
move.
were handed down from the ancient Greeks.
were discovered by Kepler in ancient Babylonian
texts.
are now known to be grossly incorrect.
Kepler’s three laws
A.
B.
C.
D.
E.
A4.5
were initially derived from fundamental physics
laws.
were initially found from observation and are
empirical rules that describe how the planets
move.
were handed down from the ancient Greeks.
were discovered by Kepler in ancient Babylonian
texts.
are now known to be grossly incorrect.
Two planets are in circular orbits around similar stars.
Each is the same distance from its parent star. One
planet is Jupiter’s size, whereas the other is Earth’s
size. Which of the following is true?
A.
B.
C.
D.
E.
Q4.7
The Jupiter-size planet has a much longer period
than the Earth-size planet.
The Earth-size planet has a much longer period
than the Jupiter-size planet.
Both planets have the same period.
The Earth-size planet has a period exactly twice
that of the Jupiter-size planet.
Not enough information is given to determine the
correct answer.
Two planets are in circular orbits around similar stars.
Each is the same distance from its parent star. One
planet is Jupiter’s size, whereas the other is Earth’s
size. Which of the following is true?
A.
B.
C.
D.
E.
A4.7
The Jupiter-size planet has a much longer period
than the Earth-size planet.
The Earth-size planet has a much longer period
than the Jupiter-size planet.
Both planets have the same period.
The Earth-size planet has a period exactly twice
that of the Jupiter-size planet.
Not enough information is given to determine the
correct answer.
Which of the following objects has zero acceleration?
A.
B.
C.
D.
E.
Q4.9
A car just starting out from a stop light
A car traveling at 70 miles per hour on a long
straight interstate highway
A car exiting an interstate on a curved off-ramp
A car slowing down for a red light
A car skidding to a stop to avoid an accident
Which of the following objects has zero acceleration?
A.
B.
C.
D.
E.
Q4.9
A car just starting out from a stop light
A car traveling at 70 miles per hour on a long
straight interstate highway
A car exiting an interstate on a curved off-ramp
A car slowing down for a red light
A car skidding to a stop to avoid an accident
The tidal force of the Moon on the Earth occurs
because
A.
B.
C.
D.
E.
Q4.14
the Moon pulls more strongly on the near side of
the Earth than the far side of the Earth.
the Moon pulls more strongly on the far side of
the Earth than the near side of the Earth.
the Moon pulls equally strongly on the near side
and the far side of the Earth.
the Moon keeps its same face toward the Earth.
the Moon is in a synchronous orbit.
The tidal force of the Moon on the Earth occurs
because
A.
B.
C.
D.
E.
A4.14
the Moon pulls more strongly on the near side of
the Earth than the far side of the Earth.
the Moon pulls more strongly on the far side of
the Earth than the near side of the Earth.
the Moon pulls equally strongly on the near side
and the far side of the Earth.
the Moon keeps its same face toward the Earth.
the Moon is in a synchronous orbit.
Key Ideas
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Copernicus’s Heliocentric Model: Copernicus’s
heliocentric (Sun-centered) theory simplified the general
explanation of planetary motions.
In a heliocentric system, the Earth is one of the planets
orbiting the Sun.
A planet undergoes retrograde motion as seen from
Earth when the Earth and the planet pass each other.
The sidereal period of a planet, its true orbital period, is
measured with respect to the stars. Its synodic period is
measured with respect to the Earth and the Sun (for
example, from one opposition to the next).
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Apparent Motions of the Planets: Like the Sun and
Moon, the planets move on the celestial sphere with
respect to the background of stars. Most of the time a
planet moves eastward in direct motion, in the same
direction as the Sun and the Moon, but from time to time
it moves westward in retrograde motion.
The Ancient Geocentric Model: Ancient astronomers
believed the Earth to be at the center of the universe.
They invented a complex system of epicycles and
deferents to explain the direct and retrograde motions of
the planets on the celestial sphere.
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Kepler’s Improved Heliocentric Model and Elliptical
Orbits: Copernicus thought that the orbits of the planets
were combinations of circles. Using data collected by
Tycho Brahe, Kepler deduced three laws of planetary
motion.
(1) the orbits are in fact ellipses
(2) a planet’s speed varies as it moves around its
elliptical orbit
(3) the orbital period of a planet is related to the size of
its orbit.
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Evidence for the Heliocentric Model: The invention of
the telescope led Galileo to new discoveries that
supported a heliocentric model. These included his
observations of the phases of Venus and of the motions
of four moons around Jupiter.
Newton’s Laws of Motion: Isaac Newton developed
three principles, called the laws of motion, that apply to
the motions of objects on Earth as well as in space.
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(1) the tendency of an object to maintain a constant
velocity,
(2) the relationship between the net outside force on an
object and the object’s acceleration,
and (3) the principle of action and reaction.
These laws and Newton’s law of universal gravitation
can be used to deduce Kepler’s laws. They lead to
extremely accurate descriptions of planetary motions.
The mass of an object is a measure of the amount of
matter in the object. Its weight is a measure of the force
with which the gravity of some other object pulls on it.
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In general, the path of one object about another, such as
that of a planet or comet about the Sun, is one of the
curves called conic sections: circle, ellipse, parabola, or
hyperbola.
Tidal Forces: Tidal forces are caused by differences in
the gravitational pull that one object exerts on different
parts of a second object.
The tidal forces of the Moon and Sun produce tides in
the Earth’s oceans.
The tidal forces of the Earth have locked the Moon into
synchronous rotation.