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Biomechanical
Principles to be Investigated
Bernouilli’s Principle and the
Magnus effect
Purpose
An
individual or object that leaves the ground and moves freely through the
air is termed a “projectile”.
The flight path of a projectile is affected by the downward pull of
gravity, lift forces, drag forces, the magnitude of the initial velocity
imparted to the projectile, the angle of attack and the location at which
the initial velocity was imparted to the individual or object with respect
to its center of mass (i.e., was a spin imparted to the projectile?).
The mass of the projectile dictates how much effort must be
expended to launch it into the air.
Whereas, the size, shape and orientation of the projectile in
flight will dictate the extent of the lift vs. the drag forces.
Why?
This lab will investigate the amount of drag and lift forces
affecting the flight path of a volleyball during two different types of
serves – a floating serve and a top spin serve.
Rationale
Bernouilli’s Principle expresses the inverse relationship between the
flow velocity at any given location on a projectile and the amount of
pressure exerted against that location.
It states that “Where the flow velocity is fast, the pressure is
low. Where the flow velocity
is slow, the pressure is high.” A
lifting force is created by a difference in pressure applied to opposite
sides of a projectile. The
direction of its application is perpendicular to the air flow moving past
the projectile.
Unlike the uniform shape of most aircraft wings, which are designed to
minimize drag forces and maximize lift forces at take-off; sports
implements vary in shape, composition, texture and size.
Minimization of drag forces when launching sports projectiles is
typically a desired outcome during most throwing, striking or kicking
movement patterns. However,
this requires spinning the projectile to stabilize its flight path and an
attack angle unique to each sport.
For example, a top spin volleyball serve enhances the lift force in the
direction of the spin [i.e., ascent phase (forward and downward),
mid-flight (downward) and descent phase (backward and downward)] while
decreasing the drag forces encountered in flight.
Imparting spin to a projectile to cause a lift force in a desired
direction is referred to as the Magnus effect.
The spin induced lift force is referred to as the Magnus force.
An
exception to the often employed attempt to minimize drag forces at launch
and during the flight of a projectile can be observed when a floating
serve is used. Spin is not imparted to the volleyball and the drag forces
exerted against it in flight create an erratic flight path. The most effective volleyball servers are those who vary the
use of floater and top spin serves to keep the opposition off-guard.
References
Kreighbaum, E. & Barthels, K.M. (1996).
Biomechanics (4th ed.). Boston:
Allyn and Bacon
Widule,
C.J. (1974). An Analysis
of Human Motion, 156-176. Lafayette,
IN: Balt. |
