In Orbit
Overview
Space Science
The celestial bodies of our solar system move according to rules
governed by gravity, Newton’s Laws of Motion, and mathematics.
In this computer-based simulation developed by the University of
Colorado, students have the opportunity to experiment with a limited
number of variables that impact the movement of planets around a
star. By manipulating the mass of objects and their velocities, students
can observe the changes in elliptical motion of these objects relative
to their gravitational foci. These variables add up to the challenges
and outcomes in launching an object into orbit.
Physical Science
Objectives
•Most objects in the solar
system are in regular and
predictable motion. Those
motions explain such
phenomena as the day, the
year, phases of the moon,
and eclipses.
In the course of completing this lesson, students should:
• Recognize how the mass of two bodies affects the gravitational
pull between them
• Recognize orbital patterns based on mass and velocity
• Reinforce their understanding of mass vs. weight
Mathematics
Time Required
90 minutes
Standards
Addressed
•Gravity is the force that
keeps planets in orbit
around the sun and
governs the rest of the
motion in the solar system.
Gravity alone holds us to
the Earth’s surface and
explains the phenomena of
the tides.
•The Earth is the third
planet from the sun in a
system that includes the
moon, the sun, eight other
planets and their moons,
and smaller objects, such
as asteroids and comets.
The sun, an average star,
is the central and largest
body in the solar system.
In Orbit
[ 1 ]
Background & Connection to the ISS
Newton’s Laws of Motion are at the heart of many physical forces that
are experienced on Earth. Moreover, the International Space Station
(ISS) provides an excellent opportunity to introduce and investigate
Newton’s Laws of Motion via orbits. The ISS weighs nearly one million pounds (more than 450 metric tons) and is essentially free falling
around Earth. To maintain this continuous free fall, or orbit, the ISS
has to maintain a velocity of approximately 17,500 miles per hour
(28,000 km per hour). This velocity is adjusted to maintain an orbiting altitude that maximizes fuel efficiency — and to keep the ISS from
crashing to the Earth’s surface or gaining too much altitude and heading off into space! The primary force that keeps the ISS in orbit is the
gravitational force of Earth.
A simulation website developed by the University of Colorado provides
a key element of this lesson plan. The site is part of the “PhET” initiative, which gets its name from Physics Education Technology project,
though it has since branched into simulations for other science disciplines. [More information on the website is available here: http://phet.
colorado.edu/en/faqs] There are a number of options for running the
simulation with your students. It is a good idea to explore the site first
and determine how best to integrate the simulation within the lesson.
Materials
Required
•A computer connected to
the Internet per student/
group
•Student handout [included]
Newton’s Laws
of Motion:
First law: An object
at rest remains at rest
unless acted on by
a force. An object in
motion will remain in
motion with the same
velocity unless acted
upon by a force.
Second law: Force acting
on a mass produces
acceleration. The greater
the mass of an object,
the greater the amount
of force is required to
accelerate the object.
Third law: For every
action, there is an equal
and opposite reaction.
In Orbit
[ 2 ]
Activity Steps
Inquiry
1. Prompt students to think about what they already know about orbits/orbiting. Ask questions such as:
• Why do we have 365 days in a year? (Why is there a leap year every four years?) It takes
approximately 365 days (365 and ¼ days, thus a leap year) for the Earth to orbit the sun.
• Why does the moon’s cycle between full moons/new moons take approximately 28 days? It takes
approximately 28 days (27.3 days) for the moon to orbit the Earth.
• How do satellites travel (or stay) in space? They orbit the Earth from varying altitudes.
2. Whether it’s through discussion or through writing, students should realize the common thread
among the questions is the subject of orbiting. Ensure students have at least a rudimentary
understanding of the concept of orbiting.
3. Share the multimedia definition of “orbit” in the Learn the Lingo feature of the CASIS Academy
website [www.casisacademy.org]. Additionally, the multimedia definitions for “microgravity” and
“satellites” help illustrate aspects of orbiting and Newton’s Laws of Motion.
4. As a transition into the main part of the lesson, ask students how satellites orbit the Earth, or how
Newton developed his ideas around motion long before vehicles were launched into space. Students
should be able to articulate that it was through observing orbits in our solar system — that it is
orbiting that keeps the universe organized and functioning. This lesson is a chance to delve deeper
into this concept.
Investigate & Manipulate
5. Divide students into small groups, so that each group has access to a computer with Internet access.
(Note: you may want to divide students earlier so their groups can view the multimedia definition(s)
in Step 3 above.)
6. Guide students through the following experience on this website:
http://phet.colorado.edu/en/simulation/my-solar-system
• Click “Run Now” — unless you’re able to download it.
• Move the slider all the way to accurate, click on the tape measure and the grid. (mid/lower right of
interface)
• Click the radio button for 4 objects (lower left of interface) and run the simulation until the purple
planet (body 2) has made one complete orbit (one year). (Students need to click Stop.)
• After the first orbit (year), turn off the traces (deselect “Show Traces” box) and watch another orbit
(year) of the purple planet (body 2).
In Orbit
[ 3 ]
7. Pose the following question to students (have them answer aloud or write their responses):
Is blue moon (body 3) circling the yellow sun (body 1) or the purple planet (body 2)?
Explain your answer.
8. Next, have students click the RESET button on the screen and prompt them to:
• Increase the mass of the sun (body 1) to 400 and allow the simulation to run for one complete
orbit of the purple planet (body 2).
• Decrease the mass of the sun (body 1) to 175 and allow the simulation to run for one complete
orbit of the purple planet (body 2). (~90 seconds)
9. Pose the following questions to students (have them answer aloud or write their responses):
How do the orbits of the planets change when the mass of the sun is increased or decreased?
Why? Explain your answer.
Why does the sun (body 1) follow a circular path? How does the path change as its mass changes?
Why? Explain your answer.
10. Have students continue manipulating data on the website and answering questions that follow the
table below:
• Choose the preset for Sun and Planet from the pull-down menu.
• Complete the data table below by changing the mass as shown and recording the length of the
year in seconds, and also measuring the distance from the planet to the sun at the closest point
(perihelion) and farthest point (aphelion). (Make sure slider is set to most accurate)
Mass of Sun
(body 1)
Mass of Planet
(body 2)
200
10
400
10
600
10
800
10
1000
10
150
10
200
1
200
20
200
50
200
100
200
200
Time of One Orbit Closest Distance Farthest Distance
(planetary year) to Sun (perihelion) to Sun (aphelion)
In Orbit
[ 4 ]
Q: When is the planet moving fastest? Why?
Q: What makes the length of the year increase and decrease? Why?
Q: A planet in a circular orbit would always be the same distance from the sun. What happens in
orbits with the shortest years? Why?
•Choose the ellipses preset from the pull-down menu.
•Move the slider bar about 2/3 of the way towards fast for this simulation.
•Run the simulation until the green planet (body 4) returns to its starting point (one planetary year)
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
•Change the y velocity of the blue planet (body 3) to 90 and the green planet (body 4) to 70.
•Run the simulation again until the green planet (body 4) returns to its starting point (one
planetary year)
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
estimate
Q: How does the year of a planet closer to the sun compare with one that is farther away? Why?
Q: How can an orbit be made more circular? Explain your answer.
Q: Describe what an orbit is and what factors affect the size, speed and time (period) of an orbit.
In Orbit
[ 5 ]
Extensions & Modifications
• One of the Apps highlighted in the Check Out These Cool Apps! feature of the CASIS Academy
website is a tracking App for the ISS. If you can access the App, consider starting the lesson that
way.
• Give students the opportunity to have a more open-ended experience in which they use this web
resource to manipulate data and try to detect patterns or seeming anomalies.
• Extend the lesson into language arts by having students write explanatory essays that teach
readers about orbits/orbiting.
• To abridge the lesson, you can complete the online prompts and manipulation as a class on your
smart board. Your familiarity with the website and with the steps should greatly speed up the
process.
Attribution
Adapted from Chris Cochran’s Orbits-Effects of Mass and Distance,
https://phet.colorado.edu/en/contributions/view/3015
In Orbit
[ 6 ]
Orbit! Or... Is It a Bit Too Confusing?
The Earth orbits the sun every 365 days. Technically, it’s every 365 and ¼ days — that’s why there is
a Leap Year every four years to make up for the quarter day. The moon orbits the Earth roughly every 28
days (27.3 to be exact). Gravity is the key to those orbits. Thanks to gravity, smaller bodies are attracted
to larger bodies. The Earth orbits the sun because the sun is much bigger, so its gravitational pull keeps
us in its orbit. Our moon orbits the Earth because our home planet is much bigger than the moon, so the
Earth’s gravitational pull keeps it in orbit.
In this activity, there is a website where the forces that affect orbits by changing the size of the objects or
the velocity of the orbiting object be manipulated. Don’t worry. It’ll make more sense once the website is
accessed. Speaking of …
1. Go to this website: http://phet.colorado.edu/en/simulation/my-solar-system
• Click “Run Now” — unless it’s already downloaded.
• Move the slider all the way to accurate, click on the tape measure and the grid. (mid/lower right
on screen)
• Click the radio button for 4 objects (lower left on screen) and run the simulation until the purple
planet (body 2) has made one complete orbit (one year). (Click Stop.)
• After the first orbit (year), turn off the traces (deselect “Show Traces” box) and watch another orbit
(year) of the purple planet (body 2).
2. Answer this question:
Is blue moon (body 3) circling the yellow sun (body 1) or the purple planet (body 2)?
Explain your answer.
3. Next, click the RESET button on the screen and: • Increase the mass of the sun (body 1) to 400 and allow the simulation to run for one complete
orbit of the purple planet (body 2).
• Decrease the mass of the sun (body 1) to 175 and allow the simulation to run for one complete
orbit of the purple planet (body 2). (approximately 90 seconds)
4. Now, do your best to answer these questions:
How do the orbits of the planets change when the mass of the sun is increased or decreased?
Why? Explain your answer.
Why does the sun (body 1) follow a circular path? How does the path change as its mass changes?
Why? Explain your answer.
InIn
Orbit
Orbit[HO-1A]
[ 7 ]
5. Continue to manipulate data on the website and answer the questions that follow the table:
• Choose the preset for Sun and Planet from the pull-down menu.
• Complete the data table below by changing the mass as shown and recording the length of the
year in seconds, and also measuring the distance from the planet to the sun at the closest point
(perihelion) and farthest point (aphelion). (Make sure slider is set to most accurate)
Mass of Sun
(body 1)
Mass of Planet
(body 2)
200
10
400
10
600
10
800
10
1000
10
150
10
200
1
200
20
200
50
200
100
200
200
Time of One Orbit Closest Distance Farthest Distance
(planetary year) to Sun (perihelion) to Sun (aphelion)
Q: When is the planet moving fastest? Why?
Q: What makes the length of the year increase and decrease? Why?
Q: A planet in a circular orbit would always be the same distance from the sun.
What happens in orbits with the shortest years? Why?
• Choose the ellipses preset from the pull-down menu.
• Move the slider bar about 2/3 of the way towards fast for this simulation.
• Run the simulation until the green planet (body 4) returns to its starting point (one planetary year)
InIn
Orbit
Orbit[HO-1B]
[ 8 ]
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
• Change the y velocity of the blue planet (body 3) to 90 and the green planet (body 4) to 70.
• Run the simulation again until the green planet (body 4) returns to its starting point (one
planetary year)
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
estimate
Q: How does the year of a planet closer to the sun compare with one that is farther away? Why?
Q: How can an orbit be made more circular? Explain your answer.
Q: Describe what an orbit is and what factors affect the size, speed and time (period) of an orbit.
InIn
Orbit
Orbit[HO-1C]
[ 9 ]