WEBELOS

Scientist

Technology Group

Ping Pong Ball Curves

Circle Ten Council

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 tube 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 inside 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.

Musical Tube

Description: A whirling tube makes musical notes demonstrating the Bernoulli Theory.

Materials:

Corrugated flexible plastic tube (Corrugated tubing is available from swimming pool supply stores. Ask for a piece about 1 meter long. Tubing is also available from toy stores under names such as Whirl-A-Tune TM)

Procedure:

1.       Hold the tube at one end and twirl the other end rapidly through the air. Make sure not to hit anything with the whirling end. A musical note will be produced.

2.       Whirl the tube at different speeds. What happens to the pitch? Why do you think this happens?

3.       Plug the end of the tube in your hand with a cloth and spin the tube. Is a sound produced? Why or why not?

Discussion:

The musical tube provides an audible demonstration of the Bernoulli Theory. The free end of the tube moves through the air much more rapidly than the end in your hand. Consequently, the velocity of the air around the free end is much greater than the velocity around the end in your hand.

Bernoulli's Theory, in general terms, describes the relationship in a fluid between pressure and velocity. Where the velocity is greater, the pressure is smaller and vice versa. The velocity of the air around the moving end of the tube is greater and therefore the air pressure there is smaller than at the slowly moving end. Inside the tube, the air is relatively stationary. However, a pressure differential is created between the two ends and air flows from the slowly moving end to the fast moving end where it spills out. The tube's corrugations cause the air to vibrate as it travels from one end of the tube to the other. The vibration produces the musical note. When the tube is moving faster, the vibration frequency increases raising the pitch. When the tube is plugged, no air flows and the sound is stopped.

The musical tube can be used to demonstrate the same pressure changes that also take place around an airplane's wing. By making air flow faster over the top of a wing than below it, a major share of aerodynamic lift is produced because the pressure on the bottom of the wing where the air is moving slower is greater than the pressure on the top of the wing where the air is moving faster. Thus the wing is pushed upwards by the difference in pressure. This is lift.

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