Universal Law of Gravity, Orbits, and Weightlessness

1
Universal Law of Gravity, Orbits,
andWeightlessness

• Chapter 9 pages 161-168 ONLY
• Also part of chapter 10, pages 192-195

2
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

3
PRETEST Astronauts inside the Space Shuttle
float around as it orbits the Earth because
(select all that apply)

1. there is no gravity in space.
2. they are falling in the same way as the Space
   Shuttle.
3. they are above the Earths atmosphere.
4. there is less gravity inside the Space Shuttle.
5. 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.
• Weightless scenes in Apollo 13 were filmed this
  way!
• Read about this on pages 166-168.
• When you read
• Keep apparent weight in mind
• Apparent weight is what you feel.
• Weight is what gravity really does.

5
Falling Around
Newtons original drawing from Principia

• Newtons cannon drawing.
• Gravitys acceleration can change an objects
  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.
6
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
• 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.

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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.

8
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 gravitys force depends
  on three things.
• The mathematical relationship is calledan
  Inverse Square Law and it looks like this
• FOR ANY 2 OBJECTS (any ? Universal)

G is a number you look up in a book. Well ignore
it. (Usually)
See pages 161-162, 165-166. Can skip Universal
Gravitational Constant, G if you like on pages
163-164.
11
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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13
Is there gravity on you if you were in the empty
space between the planets?

1. Yes
2. No

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 Newtons third law
  with the law of gravity

17
Who do you agree with?

1. Student 1
2. Student 2
3. Both are right
4. Both are wrong
5. Some of each are right

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Forces, Newtons 3rd law, Gravity

• Student 1 is correct.
• Newtons 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.

19
Does the gravitational force between Moon Earth
change if Moon doubles its mass?

1. Yes
2. No

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20
Which of the following would make you weigh half
as much as you do right now? Select all that
apply.

1. Take away half of the Earths atmosphere.
2. Double the distance between the Sun and the
   Earth.
3. Make the Earth spin twice as fast.
4. Take away half of the Earths 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?

1. It would be four times as strong
2. It would be twice as strong
3. It would be the same
4. It would be half as strong
5. It would be 1/4th as strong

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22
If the Earth shrank to half its current size but
didnt lose mass, what would gravity be on the
new surface?

1. It would be four times as strong
2. It would be twice as strong
3. It would be the same
4. It would be half as strong
5. It would be 1/4th as strong

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If the Earth grew to 3 times its current size but
didnt change mass, how would surface gravity
change?

1. It would be 9 times as strong
2. It would be 6 as strong
3. It would be 3x
4. It would be the same
5. 1/9 as strong
6. 1/6 as strong
7. 1/3 as strong

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Real triple-size planets

• We just said planets that are 3x the Earths size
  have _________ gravity.
• Do you think real planets that are 3x bigger than
  Earth have weaker gravity?
• Why not?

25
If the Moon got twice as heavy, would your
current weight (in this room) change noticeably?

1. Yes
2. No

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26
If the Earths 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
• Students know the relationship between the
  universal law of gravitation and the effect of
  gravity on an object at the surface of Earth.

28
Masses

• Earths 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 doesnt? clicker question first.
• continued soon this slide will reappear
  shortly

29
Which force is stronger?

1. Earth pulling down on pen
2. Pen pulling up on Earth
3. Same

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Masses

• Earths 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 doesnt? 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.

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Which is closer to you?

1. New York City
2. The top of Earths atmosphere
3. They are the same distance

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Gravitys 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.
• Most satellites are 200-300 miles up
• For a total distance of
• Has the distance changedby a big percentage?
• That 5 distance change results in 10 weaker
  gravity
• (How high up does the atmosphere go?)

33
Astronauts inside the Space Shuttle float around
as it orbits the Earth because (select all that
apply)

1. there is no gravity in space.
2. they are falling in the same way as the Space
   Shuttle.
3. they are above the Earths atmosphere.
4. there is less gravity inside the Space Shuttle.
5. 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

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Summary

1. Falling around
2. Weightlessness
3. Newtons Law of Gravity, examples
4. Expect questions on the next test like those we
   did here.