Universal Law of Gravity, Orbits, andWeightlessness

Chapter 9 – pages 161-168 ONLYAlso 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 _________________ __________________________________• 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

• Newton’s cannon drawing.• Gravity’s __________

_____________________ ___________

• See pages 192-195 in chapter 10 for more info.

These cannonballs ____________ ____________________________ _________________________.

Put your drawing here:

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.

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.

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0 1. True2. False

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Is there gravity on the Moon?

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0 1. Yes2. 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?

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1. Earth exerts stronger force2. Moon exerts stronger force3. Forces are the same

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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0 1. Student #12. Student #23. Both are right4. Both are wrong5. 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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0 1. It would be four times as strong2. It would be twice as strong3. It would be the same4. It would be half as strong5. 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?

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0 1. Yes2. No

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If the Earth’s atmosphere went away, would your weight change noticeably?

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0 1. Yes2. 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?

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0 1. New York City2. The top of Earth’s atmosphere3. They are the same distance

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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 …

• Has the distance changedby a big percentage?

• That ________________________________________• (How high up does the atmosphere go?)

2dist

ma Earthgrav

Moon To scale, ___________ ___________________.

Summary1. Falling around2. Weightlessness3. Newton’s Law of Gravity, examples4. Expect questions on the next test like those

we did here.