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

Orbital Forces 

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

Key Words



centripetal force 




tennis ball on ribbon 

pencil and paper



 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. What happens when you let go?


Hold the free end of the ribbon and swing the ball in a circle.


In this activity, the centripetal force on the ball is produced by ribbon tension. That is, the ribbon transmits the centripetal force which pulls the ball into a circular path. For a satellite orbiting Earth, there is no ribbon connection! What produces the centripetal "towards the center of the Earth" force?

Let go of the ribbon and watch which direction the ball travels. The ribbon makes it easier to follow the direction of the ball.


Where did the ball go?

The circle shown on the left represents the path of the ball while you were swinging it with the ribbon. Draw an arrow from the circle to represent the path of the ball when you released it. Explain your answer.


Go to Orbital Forces Teacher pages.

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This activity has been copied, with permission, from the University of Hawaii's School of Ocean & Earth Science & Technology server to ours, to allow faster access from our Web site. We encourage you to explore the original site.

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