To explore satellite orbits and gravitational force.
In this lesson, students refine and expand their understandings of gravity, forces, and motion in the context of satellite orbits. The middle-school grades are the appropriate time to more fully develop the force/motion relationship for students. However, these concepts need to be built over time. Students may view the phenomenon of a falling body as a natural occurrence which requires no further explanation. This lesson is most appropriate for students who have already had instruction in the forces acting on falling bodies and gravity in lessons such as Exploring Pendulums.
Students should already know that the earth's gravity pulls any object toward it without touching it. (Benchmarks for Science Literacy, p. 94.) This lesson will help students to more fully develop their understanding of the relationship between force and motion. "Students have no trouble believing that an object at rest stays that way unless acted on by a force. The difficult notion is that an object in motion will continue to move unabated unless acted on by a force." (Benchmarks for Science Literacy, p. 90.)
You can read Microgravity Science Primer for background information on gravity and satellite orbits.
Begin with a brief discussion to tap into existing student knowledge and misconceptions about satellites. Use questions such as these:
- Why do we send satellites into space?
- How are satellites launched?
- How do satellites stay in orbit?
- Do they ever fall out of orbit?
- Do they ever get lost?
In this part of the lesson, students explore resources from the Satellites and Space Weather page to develop an understanding of how satellites work, especially how they stay in orbit. Using the Satellite Orbits student esheet, they will refer to these pages:
- What Does Falling Have to Do with Orbits?
- Satellite Menagerie
- The Heart of Satellite Orbits
- Atmospheric Drag
- Why Do Satellites Stay in Orbit?
After students have explored these resources on their own, review the concepts with the whole class. Allow students to ask questions about things that they don't fully understand. Some common misconceptions about satellites can be found on Misconceptions About Satellites and Weightlessness. You can use this to help identify and address lingering misconceptions in students' ideas about satellites.
Using the student esheet to guide them, students will explain, in their own words, how a satellite stays in orbit around the earth and what, if anything, might cause it to fall.
Students should be able to explain that falling and forward motion combine to produce a curved path. These forces act independently of one another. A satellite orbiting around the earth may fall forever towards the earth's surface but since the earth's surface curves away at the same rate, the satellite remains suspended forever at the same height above the surface. Frictional forces on the satellite (such as atmospheric drag), however, can cause the forward speed of the satellite to decrease, changing the curvature of its orbit. The curvature is decreased so that it no longer matches the curvature of the earth's surface at the satellite's altitude, causing it to fall.
Around the World is an activity in which students build a model of an earth satellite. It can be used to extend the ideas in this lesson or as an authentic assessment of student understanding of the concepts covered in this lesson.
Microgravity in the Classroom is a NASA educational activity designed to demonstrate freefall in the classroom. This activity is particularly helpful if you find that students need more experiences with these phenomena.
Newton's Laws of Motion: Force and Inertia can be used to review the laws of motion.