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After completing this chapter, you
will be able to:
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- Explain the ways in which the composition and flow characteristics
of a fluid affect fluid forces.
- Define buoyancy and explain the
variables that determine whether a human body will float.
- Define drag, identify the components
of drag, and identify the factors that affect the magnitude of each
component.
- Define lift and explain the ways
in which it can be generated.
- Discuss the theories regarding propulsion of the human body
in swimming.
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Why are there dimples in a golf ball? Why are some people able
to float while others cannot? Why are cyclists, swimmers, downhill
skiers, and speed skaters concerned with streamlining their bodies
during competition?
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Both air and water are fluid mediums that exert forces on bodies
moving through them. Some of these forces slow the progress of a
moving body; others provide support or propulsion. A general understanding
of the actions of fluid forces on human movement activities is an
important component of the study of the biomechanics of human movement.
This chapter introduces the effects of fluid forces on both human
and projectile motion.
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Although in general conversation the term fluid is
often used interchangeably with the term liquid, from
a mechanical perspective, a fluid is
any substance that tends to flow or continuously deform when acted
on by a shearforce (42). Both gases and liquids are fluids
with similar mechanical behaviors.
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Because a fluid is a medium capable of flow, the influence of
the fluid on a body moving through it depends not only on the body’s
velocity but also on the velocity of the fluid. Consider the case of
waders standing in the shallow portion of a river with a moderately
strong current. If they stand still, they feel the force of the
current against their legs. If they walk upstream against the current, the
current’s force against their legs is even stronger. If
they walk downstream, the current’s force is reduced and
perhaps even imperceptible.
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When a body moves through a fluid, the relative
velocity of the body with respect to the fluid influences the
magnitude of the acting forces. If the direction of motion is directly
opposite the direction of the fluid flow, the magnitude of the velocity
of the moving body relative to the fluid is the algebraic sum of
the speeds of the moving body and the fluid (Figure 15-1).
If the body moves in the same direction as the surrounding fluid,
the magnitude of the body’s velocity relative to the fluid
is the difference in the speeds of the object and the fluid. In
other words, the relative velocity of a body ...