Download 3.2 Gravity and the Solar System

Unit 3 Lesson 2 Gravity and the Solar System
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Unit 3 Lesson 2 Gravity and the Solar System
Florida Benchmarks
•  SC.8.N.1.4 Explain how hypotheses are valuable
if they lead to further investigations, even if they
turn out not to be supported by the data.
•  SC.8.N.1.5 Analyze the methods used to develop
a scientific explanation as seen in different fields
of science.
•  SC.8.N.1.6 Understand that scientific
investigations involve the collection of relevant
empirical evidence, the use of logical reasoning,
and the application of imagination in devising
hypotheses, predictions, explanations and models
to make sense of the collected evidence.
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Unit 3 Lesson 2 Gravity and the Solar System
Florida Benchmarks
•  SC.8.N.1.6 Understand that scientific
investigations involve the collection of relevant
empirical evidence, the use of logical reasoning,
and the application of imagination in devising
hypotheses, predictions, explanations and models
to make sense of the collected evidence.
•  SC.8.N.2.2 Discuss what characterizes science
and its methods.
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Unit 3 Lesson 2 Gravity and the Solar System
Florida Benchmarks
•  SC.8.E.5.4 Explore the Law of Universal
Gravitation by explaining the role that gravity
plays in the formation of planets, stars, and solar
systems and in determining their motions.
•  LA.6.2.2.3 The student will organize information
to show understanding (e.g., representing main
ideas within text through charting, mapping,
paraphrasing, summarizing, or comparing/
contrasting).
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Unit 3 Lesson 2 Gravity and the Solar System
Gravity
What is gravity?
•  Gravity is a force of attraction between objects
that is due to their masses and the distances
between them.
•  Every object in the universe pulls on every other
object.
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Unit 3 Lesson 2 Gravity and the Solar System
What is gravity?
•  Objects with greater masses have a greater force
of attraction than objects with lesser masses
have.
•  Objects that are close together have a greater
force of attraction than objects that are far apart
have.
•  Gravity is the weakest force in nature, yet it is one
of the most important forces in the universe.
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Unit 3 Lesson 2 Gravity and the Solar System
What is gravity?
•  Gravity accounts for the formation of planets,
stars, and galaxies.
•  Gravity also keeps smaller bodies in orbit around
larger bodies.
•  An orbit is the path that a body follows as it
travels around another body in space.
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Unit 3 Lesson 2 Gravity and the Solar System
What are Kepler’s laws?
•  The 16th century Polish astronomer Nicolaus
Copernicus changed our view of the solar system.
•  He discovered that the motions of the planets
could best be explained if the planets orbited the
sun.
•  Like astronomers before him, Copernicus thought
that the planets followed circular paths around the
sun.
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Unit 3 Lesson 2 Gravity and the Solar System
What are Kepler’s laws?
•  Danish astronomer Tycho Brahe used special
instruments to accurately measure planetary
motions over a period of 20 years.
•  Using Tycho’s data, Johannes Kepler discovered
what we call Kepler’s laws of planetary motion.
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Unit 3 Lesson 2 Gravity and the Solar System
What are Kepler’s laws?
•  Upon plotting the orbit of Mars, Kepler saw that it
was a deformed circle.
•  After eight years of work, he realized that it was
an ellipse.
•  Kepler then proposed that each of the planets has
an elliptical orbit, with the sun at one focus of the
ellipse.
•  This is Kepler’s first law.
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Unit 3 Lesson 2 Gravity and the Solar System
What are Kepler’s laws?
•  When an object follows an elliptical orbit around
the sun, there is one point, called aphelion,
where the object is farthest from the sun.
•  There is also a point, called perihelion, where the
object is closest to the sun.
•  Today, we know that the orbits of the planets are
only slightly elliptical, but the orbits of objects
such as Pluto and comets are highly elliptical.
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Unit 3 Lesson 2 Gravity and the Solar System
What are Kepler’s laws?
•  Kepler found that a planet moves slower at
aphelion, sweeping out a narrow sector on the
ellipse.
•  Conversely, a planet moves faster at perihelion,
sweeping out a thick sector on the ellipse.
•  As a planet moves around its orbit, it sweeps out
equal areas in equal times. This is Kepler’s second
law.
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Unit 3 Lesson 2 Gravity and the Solar System
What are Kepler’s laws?
•  Kepler also looked at how long it took for the
planets to orbit the sun and at the sizes of their
orbits.
•  He discovered that the square of the orbital period
is proportional to the cube of the planet’s distance
from the sun.
•  This principle is Kepler’s third law.
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Unit 3 Lesson 2 Gravity and the Solar System
What is the law of universal
gravitation?
•  Using Kepler’s laws, Sir Isaac Newton became the
first scientist to mathematically describe how the
force of gravity behaves.
•  He reasoned that gravity is the force that accounts
for both the fall of an apple from a tree and the
movement of the moon around Earth.
•  In 1687, Newton formulated the law of universal
gravitation.
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Unit 3 Lesson 2 Gravity and the Solar System
What is the law of universal
gravitation?
•  The law of universal gravitation states that all
objects in the universe attract each other through
gravitational force.
•  The strength of this force depends on the product
of the masses of the objects.
•  Gravitational force is also inversely proportional to
the square of the distance between the objects.
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Unit 3 Lesson 2 Gravity and the Solar System
How does gravity affect planetary
motion?
•  If a ball is attached to a string and is swung
around, it moves in a circular path.
•  The inward force that causes an object to move in
a circular path is called centripetal force.
•  If the string breaks, the ball will move off in a
straight line. When the string is intact, the
centripetal force prevents the ball from flying off.
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Unit 3 Lesson 2 Gravity and the Solar System
How does gravity affect planetary
motion?
•  When planets orbit the sun, a force similar to
centripetal force prevents them from moving out
of their orbits and into a straight line.
•  The sun’s gravity is the force that keeps the
planets moving in orbit around the sun.
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Unit 3 Lesson 2 Gravity and the Solar System
Collapse
How did the solar system form?
•  The formation of the solar system is thought to
have begun 4.6 billion years ago when a cloud of
gas and dust collapsed.
•  This kind of cloud, from which solar systems form,
is called a solar nebula.
•  In a solar nebula, the inward pull of gravity is
balanced by the outward push of gas pressure in
the cloud.
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Unit 3 Lesson 2 Gravity and the Solar System
How did the solar system form?
•  Scientists think that an outside force, perhaps the
explosion of a nearby star, caused the nebula to
compress and contract under its own gravity.
•  The solar system formed in a single region of the
nebula, perhaps several light-years across.
•  The sun probably formed from a region that had a
mass that was slightly greater than today’s mass
of the sun and planets.
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Unit 3 Lesson 2 Gravity and the Solar System
How did the solar system form?
•  As a region of the solar nebula collapsed, gravity
pulled most of the mass toward the center of the
nebula.
•  As the nebula contracted, it began to rotate with
increasing speed and flattened out into a disk.
•  This disk, called a protostellar disk, is where the
central star, our sun, formed.
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Unit 3 Lesson 2 Gravity and the Solar System
How did the solar system form?
•  As the protostellar disk continued to contract,
most of the matter ended up in the center of the
disk.
•  Friction from matter that fell into the disk heated
its center to millions of degrees, resulting in the
fusion of hydrogen atoms into helium atoms.
•  The process of fusion released large amounts of
energy, the gas and dust stopped collapsing, and
the sun was born.
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Unit 3 Lesson 2 Gravity and the Solar System
How did the solar system form?
•  As the sun was forming, dust grains collided and
stuck together to form dust granules, which
increased in size to form meter-sized bodies.
•  Collisions between these bodies formed larger
bodies, called planetesimals, from which planets
formed.
•  The protostellar disk became the protoplanetary
disk in which the planets formed.
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Unit 3 Lesson 2 Gravity and the Solar System
How did the solar system form?
•  The inner part of the protoplanetary disk was so
hot that only rocks and metals were in solid form.
•  The collisions of rocky planetesimals in the inner
disk led to the formation of rocky, metallic
planets.
•  We call these inner planets the terrestrial planets.
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Unit 3 Lesson 2 Gravity and the Solar System
How did the solar system form?
•  In the cold, outer disk, massive planets made of
icy and rocky planetesimals may have formed.
•  The gravity of these planets was so strong that
they captured gas and other matter as they grew.
•  Therefore, these outer planets have rocky or
metallic cores and deep atmospheres of gas and
ice, and they are called the gas giant planets.
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Unit 3 Lesson 2 Gravity and the Solar System
How did the solar system form?
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