SCIENCE CONCEPT:
- The level of swimming skill varies greatly between
recreational and competitive swimmers. Even at the competitive level,
some swimmers are faster than others. The difference lies in the ability
to use the fundamental principles of fluid dynamics to one's advantage.
STUDENT OBJECTIVE:
- The student will demonstrate and observe how using
the principles of fluid dynamics can increase their ability to swim at an
optimum speed in the water.
OVERVIEW:
- The student will do a series of different types of swimming
strokes to show how fluid dynamics effects their speed dynamic in the
water. Also, the student will experience how the body positioning in the
water and leg kicking technique effects the speed that a swimmer can
achieve.
TEACHER TEXT:
- The same principles which makes the best swimmers so
fast are employed by anyone who has ever treaded water. In order to
understand how these principles of fluid dynamics apply, we must first
understand the forces acting on a swimmer. There are four primary forces
acting on a swimmer. These forces are similar to the forces acting on an
airplane. In the vertical plane, the weight of the swimmer is offset by
the buoyancy. However, since people have varying natural ability to
float, another means of overcoming the swimmers weight must be employed.
This is accomplished from the arm stroke and kick. By pressing down on
the water, an equal and opposite reaction occurs which lifts the swimmer
higher in the water. The other primary forces are the thrust or
propulsive force and the drag.
The drag can be divided into two components: pressure
drag and skin friction drag. The pressure drag comes from the frontal
area exposed to the water and the separation that occurs behind the
swimmer. This is similar to the pressure drag of the smooth golf ball.
In order to reduce the drag on the golf ball, dimples were introduced
which changed the nature of the flow from laminar to turbulent. The
turbulent flow delays separation and therefore reduces the pressure drag.
However, the flow around a swimmer is already turbulent. Therefore, a
swimmer must streamline his body to reduce the amount of separation. The
drag from the skin friction, on the other hand, increases when the
swimmer
becomes more streamlined since more surface area is exposed to the water.
This is not really a concern, as the pressure drag is dominant and,
therefore, the overall drag decreases. Generally speaking, the arm
stroke produces the majority of the thrust. The difference between
swimmers is how the arm stroke is used to produce thrust. The best
swimmers not only achieve thrust by pushing back on the water, but also
by moving their hands and arms like a propeller.
The most obvious production of thrust does come from
pushing back on the water like a paddle wheel or rowing a boat. In fact,
the straight arm pull was originally thought to be most efficient.
Recently, some people have advocated a straight-back pull stroke. This
concept was tested using a riverboat equipped with a caterpillar paddle
wheel. Unfortunately for the inventor, the boat practically stood still.
The explanation for this can be found in the following quote:
"Maximum efficiency in water is achieved by pushing
a large amount of water a short distance rather
than by pushing a small amount of water a large distance."
To fully appreciate the significance of this quote, we
should look at the most efficient arm stroke. This stroke involves
moving the arm along a curvilinear path. This way the swimmer is always
pushing back on still water. The advantage here is that the still water
offers more resistance than the water that is already moving back.
The kick provides a stabilizing effect in addition to the
propulsive force. Most swimmers only get a small amount of propulsive
force from their kick. The first way to improve the kick is to keep the
feet in the water. When a swimmers feet enter the water, a significant
amount of air enters as well. The air increases the dray as well as
reducing the propulsive effect. The best swimmers go further by moving
their feet during the kick to produce the same lift force achieved by
their hands.
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PREPARATION TIME:
- 20 minutes.
LESSON TIME:
- 90 minutes.
TEACHER PREP:
- Locate a swimming pool that your class can use. If
there isn't one at your local high school there may be a YMCA close by
that you can use.
WORDS TO KNOW:
- fluid dynamics
- buoyancy
- propulsive force
- pressure drag
- skin friction drag
- laminar flow
- turbulent flow
- thrust
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