SCIENCE CONCEPT:
- The 'dance' of the knuckleball is caused by a very slow
spin. Researchers have learned that a slight change in the orientation
of the ball with respect to the flow of air results in dramatic changes
in the forces acting on the ball. Not only does the magnitude of the
force change, but the direction also changes. This is why the ball
appears to 'dance'.
STUDENT OBJECTIVE:
- The student will do a comparison study of how the
turbulent and laminar flow overlap causing the knuckleball to 'dance'.
OVERVIEW:
- The student will make a Venn Diagram showing how the turbulent
and laminar flow change the flow of the knuckleball causing it to appear
to 'dance'. There is a point where the turbulent and laminar flow
overlap. This is called the flow asymmetry and is developed by the
stitch pattern on a baseball.
TEACHER TEXT:
- The most likely reason for the 'dance' of a knuckleball
is a very slow spin. Researchers have learned that a slight change in
the orientation of the ball with respect to the flow of air results in
dramatic changes in the forces acting on the ball. Not only does the
magnitude of the force change, but the direction also changes. This is
why the ball appears to 'dance'.
The mechanism by which the forces change magnitude and
direction is not known. However, one can theorize that the stitches play
a key role. The stitches will most likely cause the boundary layer to
trip to a turbulent state. As we know, turbulent flow will stay attached
longer than laminar flow. In fact, once the boundary layer becomes
turbulent, a separated flow tends to reattach. This reattachment will
dramatically alter the forces on the ball. Similarly, as the ball
rotates, a region that was turbulent due to the position of the stitches,
might now become laminar. The laminar flow will separate earlier than
the turbulent flow. This altering of the state of the flow from laminar
to turbulent, separated to attached, would cause the forces on the ball
to fluctuate as shown by the experiments.
Furthermore, it is important to note that even if the
pitcher throws the ball with no rotation, the flow asymmetry will cause
the ball to rotate. The flow asymmetry is developed by the stitch pattern on a
baseball.
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PREPARATION TIME:
- 10 minutes.
LESSON TIME:
- 30 minutes.
TEACHER PREP:
- Gather materials
WORDS TO KNOW:
- knucleball
- 'dance'
- boundary layer
- turbulent
- laminar
- flow asymmetry
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