ISTAT Banner

Gravity, Orbits, and Planetary Motion

Each activity or resource is labeled by access mode:

Requires computer access
Requires downloading from the Web
$$ Order materials and supplies
Main Topics & Concepts
  • Gravity is a universal force between any two masses
  • Gravity determines the force on an object that we call its "weight" on Earth.
  • The force of gravity creates centripetal acceleration that makes objects revolve about common centers of gravity.
  • Orbits are naturally elliptical.
  • Man made satellites obey the same laws, and have the same types of orbits.
This topic applies to California state standards 1e-g in 9th grade Physical Science. 

Science Probe Text
Science Probe I § 19.1 Forces
Activity Access Description
S P I:
Activity 19B The Force of  Gravity

book  Students compare weight, a measurement of force whose metric unit is the Newton, with mass, measured in kilograms.

Activities & Lessons
Activity Access Description
Gravity and Motion, Activity 3

Galileo SSI Education

web Pages on Ellipses, Gravity and Newton's laws, and Keplerian orbits, alternating with self-scored on-line quizzes.

Orbits and Escape Velocity

Strobel's Astronomy Notes

paperweb Gravity as a force of acceleration for circular motion. Worked problems are a large part of the text. Has review questions each page that could be worksheet or homework. Fairly abstract and a bit dry.
Take a Spin Through the Solar System


web  Students can calculate spin rates for the Sun, Saturn, Uranus and Jupiter using movies and data. Student worksheet structures the mathematics.

More Universe at Your Fingertips

G-4 The Sun's Period of Rotation


book Students examine images of the Sun showing sunspots taken with the Yohkoh satellite. They use an overlay grid to mark the positions of the spots and then calculate the rate at which the spots are moving to arrive at the rotational period of the Sun. 

How Big is That Star?

Imagine the Universe

paperweb  Students examine the relation of weight to radius for various stars, and then demonstrate the way a binary star's orbit can influence its brightness.
Bending Spacetime in the Basement

Fourmilab/John Walker

webpaper What looks like a very cool experiment that purports to demonstrate gravitational attraction between <1 km weights by building a torsion balance. This should be possible, given adequate care and minimal earth movements. Printable directions are illustrated, but not tested here.
Planetary and Satellite Motion

The Physics Classroom

web Tutorial lesson starts out with qualitative introduction to Kepler's laws (GIF animation of 2nd law). Teachers may wish to omit the Newtonian Gravitational treatment. The lesson ends with interactive concept questions and some numerical problems.

*Astronomical Society of the Pacific. Ordering Info $$

Resources Access Description


web Questions and answers about gravity; some basic, some more aimed at life science applications. 
Circular Motion Principles for Satellites

Physics Classroom

web Exposition on escape velocity and gravity as the centripetal force that holds a satellite in orbit. Interactive questions with feedback.
Rogue Star

University of MD

web This program lets you define the size of a star and set it cruising through the solar system. The realtime display shows how the orbits of planets evolve under the influence of the "rogue star." Good demonstration of gravity in action.
Kepler's Second Law


web A Java applet for simulating elliptical orbits. Highlights the "swept out" area, and can show velocity and acceleration vectors. Time is a little hard to control, but it can store up to 4 orbit sections with different colors.

Takes some practice to learn how to control all variables. Instructors should become comfortable before introducing the tool to students as part of an assignment.

Orbit Simulator

Explore Science

web Multi-parameter simulation of orbital motion. Allows control of number of objects, gravitational constant, initial velocities and positions. Requires a ShockWave player.
Orbits in Strongly Curved Spacetime


web This applet shows the variations in orbits produced when local spacetime is warped by a superdense black hole. The page has lots of explanation and even some general relativity towards the bottom, but the sections on how to use the applet and the basic ideas should be accessible to talented and gifted students.

** Space Station Biological  Research Project