November Cub Scout Roundtable Issue
Volume 10, Issue 4
A Cub Scout Gives Goodwill
Webelos Craftsman & Scientist
Tiger Cub Achivement #4
Ping Pong Ball Curves
Circle Ten Council
Two cardboard mailing tubes of
a greater diameter than a ping pong ball
1 sheet of medium sand paper
Ping pong ball
Cut a 2 foot length from the mailing tube.
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.
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.
Practice throwing ping-pong ball curves in an open place.
Hold the tube with your throwing hand at the end opposite the sand
paper. Drop the ping-pong ball in the tube.
Quickly swing the tube horizontally through the air. The ball will
shoot out the tube and curve through the air as it flies forward.
Repeat step 3, but use a piece of mailing tube with no sandpaper on
the inside and note the results.
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.
Description: A whirling tube makes musical notes
demonstrating the Bernoulli Theory.
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)
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
Whirl the tube at different speeds. What happens to the pitch? Why do
you think this happens?
Plug the end of the tube in your hand with a cloth and spin the tube.
Is a sound produced? Why or why not?
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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