PING PONG BALL CURVES

SUBJECT: Aeronautics
TOPIC: Lift
DESCRIPTION: A ping pong ball is tossed with a rapid spin causing it to curve in flight.
CONTRIBUTED BY: Demonstrations and Laboratory Experiences in The Science of Aeronautics, Civil Aeronautics Administration and American Council on Education (1945)
EDITED BY: Roger Storm, NASA Lewis Research Center

MATERIALS:
Two cardboard mailing tubes of a greater diameter
than a ping pong ball
1 sheet of medium sand paper
Ping pong ball
White glue
Magic Marker
Ruler
Knife

PROCEDURE:
1. Cut a 2 foot length from the mailing tube.
2. Roll up the sheet of sand paper, with the grit to the inside, into a tube. Check to see that the ping pong ball will still roll in the tupe with the sandpaper in place. Remove the sandpaper, spread white glue in several places and slide the sand paper back into the mailing tube so that the sand paper is flush with one end. Let the glue dry.
3. Use a marker to make two circles around the ball at right angles with each other. These lines will help you see the spin of the ball.

THROWING:
1. Practice throwing ping pong ball curves in an open place.
2. Hold the tube with your throwing hand at the end opposite the sand paper. Drop the ping pong ball in the tube.
3. Quickly swing the tube horizontally through the air. The ball will shoot out the tube and curve through the air as it flies forward.

4. Repeat step 3, but use a piece of mailing tube with no sandpaper on the inside and note the results.
5. Now repeat both steps 3 and 4, but use your other hand to throw the ball so that it spins in the opposite direction and note the new results.

DISCUSSION:

The mailing tube makes it easy to achieve a "major league" curve pitch with a ping pong ball. As the ball is thrown from the tube, the ball rubs against the sand paper on the side of the tube from the direction the tube is moving. Friction from the sand paper on the ping pong ball causes it to spin rapidly in a clockwise direction for right-handed throws and counterclockwise for left handed throws.

As the ball spins, the surface friction of the ball with the surrounding air drags a thin layer of air with it. This is referred to as the boundary layer. At the same time the ball spins, it is moving forward. On one side of the ball, the boundary layer air is traveling in the same direction as the air stream that is flowing around the ball (the blue arrows). On the other side, it is traveling in the opposite direction (the red arrows). On the side of the ball where the air stream and boundary layer air are moving opposite to each other friction between the two slows the air stream. On the opposite side the layers are moving in the same direction and the stream moves faster. According to Bernoulli's Principle, faster moving air exerts less pressure, so the ball is pushed and it curves to the right for right-handed throws. Left-handed throws produce a curve to the left. .

A practical application for this curving effect is a rotating cylinder flap for an airplane wing.

A rotating cylinder is mounted in the joint between a wing and its flaps. During low speed operation, the cylinder spins rapidly in the same direction as the air stream. Boundary layer air is bent down-ward over the steeply angled flap. This increases lift for the wings and delays the buildup of turbulence conditions that could lead to a stall.

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