Hawai'i Space Grant College, Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, 1996

Orbital Forces
Teacher Page

To demonstrate orbital motions and forces using a tennis ball swung by a ribbon.


A center-directed force that causes an object to follow a circular path is called a centripetal force. When you swing a tennis ball from the end of a ribbon, you must pull on the ribbon - exerting a centripetal force. When you let go of the ribbon, the ball travels in a straight path but at a right angle to the ribbon at the moment of release. In other words, the ball follows a tangential path to the circle it was orignially traveling in.

Part of Newton's first law of motion states that an object in motion will move in a straight line unless acted on by an unbalance force. In the case of the tennis ball, your inward pull on the ribbon is the unbalance force that keeps the ball traveling in a circle instead of a straight line. Upon release, the ball travels away in a straight line in the exact direction it was traveling at that very moment.

In the case of a satellite in space, the launch vehicle that carried it up to orbit aimed it in a direction parallel to the Earth's surface. According to Newton's first law, the satellite will travel in a straight line. So, why doesn't it keep traveling straight away from us? Earth's gravity acts as an unbalance force, pulling on the satellite and causing the satellite to follow a circular path.

This Activity

The tennis ball and ribbon demonstration is a good analogy of a satellite in orbit. The pull of your hand through the ribbon represents gravity.


Collect the following: one old tennis ball, one yard of one-inch-wide cloth ribbon, one dowel rod which is two inches long by 3/16 inches in diameter, sharp knife, needle and thread, stapler, and white glue.

To construct: loop one end of the ribbon over the dowel rod and glue or staple together. Hem or tie the other end of the ribbon to prevent raveling. Cut a one-ich-wide slit in the tennis ball. Then slip the dowel and ribbon at an angle through the slit in the tennis ball. The rod will prevent the ribbon from being pulled out.

Alternative Instead of using a tennis ball and ribbon, you could simply tie a string around an eraser.

In Class

Conduct this activity outside where flying tennis balls or erasers will not harm people or property.


When released, the ball will fly off on a tangent to the circle.

Go to Orbital Forces Student pages.

Return to Planetary Properties Activity Index.

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