Chapter 10: Fluid Mechanics

  • Define:
    • Relative and absolute motion
    • Buoyancy
    • Pressure
    • Fluid
    • Drag force
    • Lift force
  • Explain how a fluid exerts forces on an object moving through it
  • Identify the components of fluid forces
  • Distinguish between surface drag and form drag
  • Describe Bernoulli's Principle
  • Describe the Magnus effect
  • Identify the various factors that determine the effect fluid forces have on an object

Can humans fly?

Fluid: A liquid or gas

Buoyant force: Upward force acting on an object in a fluid that is equal to the weight of the fluid displaced by the object

Dunking a ball in the water: What determines a buoyant force?

What affects motion through a fluid?
Surface - Shape - Size - Velocity - Density

Drag Force: The component of dynamic fluid force that acts in opposition to the relative motion of the object with respect to the fluid.
Surface Drag: Drag force acting on an object within a fluid and caused by friction between the fluid and the surface of the object.
Form Drag: Drag force acting on an object within a fluid and caused by the impact forces of the fluid molecules with the object.

Drag Force: FD = ½CDρAv2

Bob Beamon's Long Jump Record (Relative and absolute motion)
How was each factor from the drag force equation modified in Bob Beamon's jump?

Drag Forces in Sport
Speed Skiing, Cycling, Running
Should grown men shave their legs?

Calculating drag in speed skiing

Terminal velocity: The ultimate speed that can be attained when falling under the influence of gravity (skydiving)

Textbook example

Lift Force
Bernoulli's Principle: Faster-moving fluids exert less pressure laterally that do slower-moving fluids.

FL = ½CL A ρ v2

Magnus Force (Tennis Ball Backspin)


Fluid forces in swimming
Swimsuit effect on performance

Example of how fluid forces affect motion of projectiles
Gravity and air density variations

Air density is based upon:

  • Air pressure
  • Temperature
  • Humidity