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With Explore the Planets, investigate the planets, their moons, and understand the processes that shape them. By G. Jeffrey Taylor, Ph.D. See our DVD collection.
The flight of this basketball pass is like the orbit of a satellite. If there was no gravity, the pass would fly straight. With gravity, the ball curves downward. A slow pass curves more than a fast pass. Satellite orbits are like very, very fast passes.
Windows to the Universe original artwork.

What does falling have to do with keeping a satellite in orbit?

The key to understanding satellite motions is in recognizing that gravity acts only in the downward direction - forward motion and falling are totally independent of each other.

If there were no gravity or frictional forces acting on the basketball in the figure, it would follow the straight line path forever. Gravity makes it drop down from this straight line path. The forward speed of the ball determines whether the path is only slightly curved or sharply curved. The distance it falls from the straight line depends on the time it takes to travel to a given spot. If the ball is thrown at high speed along the path it will get to the net rapidly and have very little time to fall along the way. If the basketball is thrown more softly, it will have time to fall quite far before arriving at the basketball net.

The relationship between forward speed and the curvature of the path is at the heart of satellite orbits.

Last modified February 2, 2010 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA