Chapter 15

After completing this chapter, you will be able to:

• 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.

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?

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.

The ability to control the action of fluidforces differentiates elite from average swimmers.

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.

### Relative Motion

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.

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 ...

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