SCIENCE CONCEPT:
- The discus is greatly influenced by aerodynamic forces.
In fact, greater distances can be achieved by throwing the discus into a
moderate headwind. This is due to the importance of the aerodynamic lift
produced by the discus in flight.
STUDENT OBJECTIVE:
- The student will illustrate on paper the Bernoulli
Principle that works in relationship to the discus using a cross section
of the discus.
OVERVIEW:
- The student will draw a cross section of the discus and
illustrate how the Bernoulli Principle works in relationship to a discus
as a symmetric airfoil.
TEACHER TEXT:
- In order to completely understand the aerodynamic lift
produced by the discus we must look at the shape of the discus. By
examining the cross section we notice that both the upper and lower
surface have the same shape. Therefore, we can consider the discus cross
section as a symmetric airfoil. If given a small angle of attack the
discus will produce lift, just like a symmetric airfoil. Again, we just
need to look at the Bernoulli Principle to see how this works. Given an
angle of attack, the stagnation point will move from the centerline of
the discus to the lower surface. Therefore, the air traveling over the
upper surface has to travel faster than the air on the lower surface.
This translates to a higher pressure on the lower surface than on the
upper surface. Hence, the production of lift. However, as is the case
with any airfoil, if the angle of attack is too large, the flow will
separate. This separation represents the sudden loss of lift. For a
discus this occurs at approximately 26 degrees angle of attack.
The velocity of the wind increases the speed of the air
traveling over the discus. This implies an increase in the lift force
experienced by the discus. The increased lift translates to longer
flight time, and hence, greater lift. Of course, this increase in
performance doesn't come without a price. The discus thrower must be
more precise with his throwing technique to take advantage of the
headwind.
The lift imparted on the discus is similar to the lift
felt by your hand when you hold it outside the window of a moving car.
Your flattened hand experiences very strong forces acting either up or
down depending on which way it is angled (giving it a positive or
negative angle of attack). The other force you feel, the one pushing
your hand back, is the drag or frictional force.
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PREPARATION TIME:
- 20 minutes.
LESSON TIME:
- 40 minutes.
TEACHER PREP:
- Gather materials. Contact a local high school to see
if they have a discus thrower who can come and give a demonstration.
WORDS TO KNOW:
- symmetric airfoil
- Bernoulli Principle
- stagnation point
- angular momentum
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