• Define:
  • Relative and absolute motion
  • Bouyancy
  • 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

"How to Throw the Goopball", by Daniel Engber.

What is a fluid?

Within a biomechanics context, what are two major fluids?

Within a biomechanics context, why do we care?

Two major fluid forces within biomechanics: Buoyant and Dynamic Fluid Forces

Buoyant force: A force that acts upward on an object, at the center of volume, as a result from its immersion in a fluid; the magnitude of the buoyant force is equal to the weight of the fluid that is displaced by the object. Consequently, buoyant forces acting on humans depend upon a couple of factors.

Density = mass(kg)/volume(m³); another measure of density is specific gravity, which is the ratio of an object's weight and the weight of the same volume of water.

With this in mind, what do you suppose the specific gravities of bone, muscle, and fat are?

And with this in mind, can you also estimate the volume of the human body?

Finally, considering the information above, how might you make it easier for you to float?

Several other interesting discussions on buoyancy for seals, penguins, and humans...

Dynamic force: A force that is due to relative motion within a fluid.

Without considering any equations, what affects motion through a fluid?

Before getting too much further into dynamic fluid forces, we should consider two other factors that are important to understand when discussing dynamic fluid forces: relative motion and laminar versus turbulent flow...

Drag Force (F_D = (1/2)C_DρAv²): The component of dynamic fluid force that acts in opposition to the relative motion of the object with respect to the fluid.

Form Drag: Drag force acting on an object within a fluid and caused by the impact forces of the fluid molecules with the object.

Surface Drag: Drag force acting on an object within a fluid and caused by friction between the fluid and the surface of the object.

Various Applications...

The Mexico City Olympic Games

Drag Forces in Sport
Speed Skiing, Cycling, Running, & Swimming

Calculating drag in speed skiing

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

 

Lift Force (F_L = (1/2)C_LρAv²)

Lift is often caused by the lateral deflection of fluid molecules as they pass by an object. The object exerts a force on the molecules that causes a lateral deflection (an acceleration due to a change in direction). According to Newton's 3rd Law, an equal but opposite force is applied to the object by the molecules. This is the lift force, and it is proportional to the magnitude of the acceleration and the mass of the molecules.

Some times, lift forces stem from Bernoulli's Principle: Faster-moving fluids exert less pressure laterally that do slower-moving fluids.

Airplane example:




 

Magnus Force

The Magnus Force is the force that results from spinning objects, such as balls.

Practical applications: Curveballs or Fastballs, Jeff Francis, and David Beckham

Powerpoint slides that may help further clarify information from this chapter.

Summary

1. Fluid forces act as objects interact with fluids (we are typically concerned with water and air). The fluid forces that were discussed included: 1) buoyant forces and 2) dynamic fluid forces.
2. The magnitude of the buoyant force is equal to the weight of the fluid displaced by the object; the buoyant force is a vertical force that is typically directed upward.
3. The dynamic fluid force is caused motion of an object within a fluid and can be resolved into two components: 1) the drag force and 2) lift force.
4. The drag force is directed against the direction of motion and is highly dependent upon the relative velocity of the object.
5. The lift force is directed orthogonal to the drag force and is also dependent upon the relative velocity of the object; lift is also dependent upon the shape, orientation, and possible spin of the object.