Orbits

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Introduction:
Gravity is a force that diminishes with distance, but does not drop to zero. When astronauts orbit the Earth, they FEEL weightless, but they have NOT "escaped" gravity. Orbital motion feels the same as drifting, but orbits REQUIRE gravity.

Activity:

 * Try all of the the PRESETS for this orbital motion simulation:
 * [[image:EarthToMars.png width="216" height="180" link="@http://phet.colorado.edu/sims/my-solar-system/my-solar-system_en.html"]]

Conclusion:
The force of gravity decreases at greater distances, so objects in orbit tend to move more slowly. When objects have a perfect match of speed and distance, they have circular orbits. When the speed and distance are not perfectly matched, the orbit is an ellipse, and the speed and distance change as the orbit progresses.