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
PRETEST: Astronauts inside the Space Shuttle float
around as it orbits the Earth because (select all that
apply)
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there is no gravity in space.
they are falling in the same way as the Space Shuttle.
they are above the Earth’s atmosphere.
there is less gravity inside the Space Shuttle.
the sky is blue.
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Weightlessness
•
•
When have YOU ever felt weightless? [ignore being in water]
Weightless when you accelerate (fall) as much as
everything around you.
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Weightless scenes in Apollo 13 were filmed this way!
Read about this on pages 166-168.
When you read:
–
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–
Keep “apparent weight” in mind
“Apparent weight” is what you feel.
“Weight” is what gravity really does.
Falling Around
Newton’s original drawing from Principia
•
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Newton’s cannon drawing.
Gravity’s acceleration can
change an object’s
direction.
See pages 192-195 in
chapter 10 for more info.
These cannonballs are falling because
of gravity from Earth.
Orbiting objects fall AROUND Earth.
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, why are they weightless?
• Draw a cartoon in your own notes that
illustrates why astronauts are weightless.
• Are they accelerating while in Earth orbit?
Clicker.
Are the astronauts accelerating while in
Earth orbit?
1. Yes
2. No
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Are the astronauts speeding up while
in Earth orbit?
1. Yes
2. No
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Universal Law of Gravity:
• Newton hypothesized that gravity’s force depends on three
things.
• The mathematical relationship is called
an Inverse Square Law and it looks like this:
• FOR ANY 2 OBJECTS (any  Universal)
Fgravitybetween 2 objects 
Gmobject1mobject2
d
2
G is a number you
look up in a book.
We’ll ignore it.
(Usually)
See pages 161-162, 165-166. Can skip “Universal
Gravitational Constant, G” if you like on pages 163-164.
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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Is there gravity on you if you were in the empty
space between the planets?
1. Yes
2. No
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Why not or from what?
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Is there such a thing as zero gravity?
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?
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Student #1
Student #2
Both are right
Both are wrong
Some of each are right
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Forces, Newton’s 3rd law, Gravity
• Student 1 is correct.
• Newton’s 3rd law says both forces must be
equal.
• The law of gravity is F = Gm1m2/d2
– It tells us HOW STRONG the two equal forces are.
Does the gravitational force between Moon &
Earth change if Moon doubles its mass?
1. Yes
2. No
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Which of the following would make you weigh half as
much as you do right now? Select all that apply.
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Take away half of the Earth’s atmosphere.
Double the distance between the Sun and the Earth.
Make the Earth spin twice as fast.
Take away half of the Earth’s mass.
There are two things, not on this list, you could do to weigh half as much.
What are they?
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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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If the Earth shrank to half its current size but didn’t lose mass,
what would gravity be on the new surface?
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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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If the Earth grew to 3 times its current size but didn’t
change mass, how would surface gravity change?
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It would be 9 times as strong
It would be 6 as strong
It would be 3x
It would be the same
1/9 as strong
1/6 as strong
1/3 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 6 x 1024 kg. 6 trillion trillion kg.
– 6,000,000,000,000,000,000,000,000 kg (24 zeroes)
– Earth is 60 billion trillion times heavier than a 220 lb person.
• When a pen falls down, why does the pen move &
the Earth doesn’t? [clicker question first].
• [continued soon – this slide will reappear shortly]
Which force is stronger?
1. Earth pulling down on pen
2. Pen pulling up on Earth
3. Same
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Masses
• Earth’s mass is 6 x 1024 kg. 6 trillion trillion kg.
– 6,000,000,000,000,000,000,000,000 kg (24 zeroes)
– Earth is 60 billion trillion 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 DOES accelerate up, VERY slowly!
• Similar to planets tugging on stars.  Found 464*
planets around other stars (since 1995**)!
– *as of 7/13/2010
– **I just saw at exoplanet.eu (NOT edu) 1 was discovered in 1989! The first
strong detection was in 1995, though.
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, this moon is WAY
too close.
Gravity’s acceleration in space
• A previous slide showed us: Fgrav = G m1 m2 / d2
• What is the numerical distance in the expression above?
• RHETORICAL QUESTION: How does this affect astronauts?
• How far are the astronauts from Earth? Draw a dot on your notes.
Space shuttle or ISS
• Most satellites are 200-300 miles up
• For a total distance of …
• Has the distance changed
by a big percentage?
Earth
• That 5% distance change results in ≈10% weaker gravity
• (How high up does the atmosphere go?)
Astronauts inside the Space Shuttle float around as it
orbits the Earth because (select all that apply)
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there is no gravity in space.
they are falling in the same way as the Space Shuttle.
they are above the Earth’s atmosphere.
there is less gravity inside the Space Shuttle.
the sky is blue.
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This slide is not in your notes
• Homework due tomorrow:
– Page 179
• 3, 9
– Pages 181-183
• 1, 2, 9 (HINT: see #10), 15,
• Not homework but worth reviewing,
since we discussed these in class:
– 10, 11, 14, 17, 18, 19, 22, 23, 30, 31, 34.
– For the 2nd test rewrite: 24, 28
Summary
1.
2.
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4.
Falling around
Weightlessness
Newton’s Law of Gravity, examples
Expect questions on the next test like those
we did here.