Hawai'i Space Grant College, Hawai'i Institute of Geophysics and
Planetology, University of Hawai'i, 1996
To demonstrate orbital motions and forces using a tennis ball swung
by a ribbon.
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
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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
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