Universal Law of Gravity, Orbits, and
Weightlessness
Chapter 9 – pages 161-168 ONLY
Also part of chapter 10, pages 192-195
Student Learning Outcomes
• Understand the conditions that lead to
weightlessness
• Give examples of weightlessness other than
objects in space
• Understand universal law of gravity and
inverse square laws
• Calculate changes in gravitational forces as
parameters change
Weightlessness
•
•
When have YOU felt weightless? [ignore being in water]
Weightless when you _________________
__________________________________
•
•
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Weightless scenes in Apollo 13 were filmed this way!
Read about this on pages 166-168.
When you read:
–
–
–
Keep “_________ weight” in mind
“___________ weight” is ______________.
__________________________________.
Falling Around
Put your drawing here:
• Newton’s cannon drawing.
• Gravity’s __________
_____________________
___________
• See pages 192-195 in
chapter 10 for more info.
These cannonballs ____________
____________________________
_________________________.
California Elementary School Science
Standards for orbits
•
From California Science Standards, grade 5:
–
•
Students know the path of a planet around the Sun
is due to the gravitational attraction between
the Sun and the planet.
And from the high school standard:
–
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Students know applying a force to an object
perpendicular to the direction of its motion
causes the object to change direction but not
speed (e.g., Earth's gravitational force causes a satellite in a
circular orbit to change direction but not speed).
Students know circular motion requires the
application of a constant force directed toward
the center of the circle.
Falling around – part 2
• Are orbiting satellites falling? Yes or No?
– Example: ISS: International Space Station.
• Are the people inside ISS falling? Y/N?
• So, ____________________________?
• Draw a cartoon in your own notes that
illustrates why astronauts are weightless.
• Are they _______________ while in Earth
orbit?
Universal Law of Gravity:
• Newton hypothesized that _____________________
_______________________________________.
• The mathematic relationship is called
an Inverse Square Law and it looks like this:
Some of the questions we’re going to do in class now have been INTENTIONALLY left
out of your notes.
Gravity is forcing you towards the person next to
you.
1. True
2. False
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Is there gravity on the Moon?
1. Yes
2. No
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The Earth is 80x more massive than the Moon. How does the
strength of the gravitational force that the Moon exerts on Earth
compare to the gravitational force that Earth exerts on the Moon?
1. Earth exerts stronger force
2. Moon exerts stronger force
3. Forces are the same
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Student debate about last question
• Student 1: I thought that whenever one object exerts a force
on a second object, the second object also exerts a force that
is equal in strength but in the other direction. So even though
Earth is bigger and more massive than the Moon, they still
pull on each other with a gravitational force of the same
strength, just in different directions.
• Student 2: I disagree. I said that Earth exerts the stronger
force because it is way bigger than the Moon. Because its
mass is bigger, the gravitational force Earth exerts has to be
bigger too. I think you are confusing Newton’s third law with
the law of gravity
Who do you agree with?
1.
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Student #1
Student #2
Both are right
Both are wrong
Some of each are right
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If you were twice as far from Earth as you are
now, how would gravity change?
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It would be four times as strong
It would be twice as strong
It would be the same
It would be half as strong
It would be 1/4th as strong
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Real triple-size planets
• We just said planets that are 3x the Earth’s
size have _________ gravity.
• Do you think real planets that are 3x bigger
than Earth have weaker gravity?
• Why not?
If the Moon got twice as heavy, would your
current weight (in this room) change
noticeably?
1. Yes
2. No
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If the Earth’s atmosphere went away,
would your weight change noticeably?
1. Yes
2. No
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California Elementary School Science
Standards for gravity
•
From California Science Standards, hgih
school
a. Students know the relationship
between the universal law of
gravitation and the effect of gravity
on an object at the surface of Earth.
Masses
• Earth’s mass is ________________________.
– _____________________________kg (__ zeroes)
– ____ is _____________ times heavier than a 220 lb person.
• When a pen falls down, why does the pen move &
the Earth doesn’t? [clicker question first].
• Earth _____________________.
• Similar to _____________________.  Found
____+ planets around other stars (since 1995)!
Which is closer to you?
1. New York City
2. The top of Earth’s atmosphere
3. They are the same distance
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Moon
To scale, ___________
___________________.
Gravity’s acceleration in space
• A previous slide showed us:
• What is the ______________ in the expression above?
• RHETROCIAL QUESTION: How does this affect astronauts?
• How far are the astronauts from Earth?
• ___________________________
• For a total distance of …
a grav 
• Has the distance changed
by a big percentage?
mEarth
dist 2
Earth
• That ________________________________________
• (How high up does the atmosphere go?)
Summary
1.
2.
3.
4.
Falling around
Weightlessness
Newton’s Law of Gravity, examples
Expect questions on the next test like those
we did here.