Biomechanics Final Exam

Drag:

a resistance force that is caused by the dynamic action of a fluid

Skin friction:

resistance from friction between adjacent layers of fluid near a body moving through the fluid

Fluid particles slowed due to

shear stress

Factors that affect Skin friction:

velocity of fluid flow, surface area, roughness, viscosity

Form drag:

resistance from a pressure differential between the lead and rear sides of a body moving through a fluid

Pressure differential: difference in pressure surrounding the

body

Factors that affect Form drag:

relative velocity, pressure gradient, surface area

Help minimize Form drag by

streamlining, cyclists drafting

Wave drag:

resistance from the generation of waves at the interface between two different fluid

Wave drag has large effect on

human swimmers

Factors that affect Wave drag:

greater up and down motion, increased swimming speed

Skin Friction:

Rough surfaces = more friction =

more resistance

Smooth surfaces = ____ drag = faster movement

reduce

Foam Drag: wide shapes = ___ drag

more

Wave Drag: More waves = ____ resistance

more

Coefficient of drag

Depends on the shape and orientation of a body

Coefficient of drag values

Index of a body's ability to generate fluid resistance (usually measured in a wind tunnel)

Center of Mass / Center of Gravity:

Point about which the sum of torques produced by the weights of the body segments is equal to 0

COG is usually about ___ of your standing height.

55-57%

COG/COM different methods

- Reaction board

- Segmental method

Reaction board:

requires a scale, a platform & rigid board with sharp supports on either end.

Segmental method:

uses data for the average locations of individual body segments, COGs as related to a percentage of segment length

The COG of a symmetrical object of homogeneous density is the

exact center of the object

When mass is not constant, the COG shifts in the direction of ____ mass.

greater

COG can be located

outside of the body

Concentric:

muscle torque and joint movement occur in the SAME direction

Eccentric:

muscle torque and joint movement occur in OPPOSITE directions

Concentric vs eccentric forces: which has a higher injury risk

Eccentric

Bernoulli's principle

Inverse relationship between relative velocity and relative pressure in a fluid flow

Bernoulli's principle - an increase in the speed of a fluid occurs simultaneously with a

decrease in pressure

Bernoulli's principle - Factors that affect it:

velocity, pressure, lift force

Magnus FORCE:

lift force created by spin

Magnus EFFECT:

deviation of the trajectory of a spinning object toward the direction of spin

- Ex: topspin, sidespin, backspin

What is buoyancy, and how does it relate to COG

- Fluid force that acts vertically upward

- COG is usually inferior to center of volume due to the lungs

COV:

point at which the buoyant force acts

Archimedes' principle:

the magnitude of the buoyant force acting on a given body is equal to the weight of the fluid displaced by the body

A foil shape capable of generating lift in the presence of

fluid flow

Angle of Attack:

angle between the longitudinal axis of the body and the direction of the fluid flow.

Angle of Attack: Steep attack =

no lift

Angle of Attack: Positive Attack =

lift capabilities

The faster the object moves = ____ lift is generated

more

Smoother surfaces reduce drag & ____ lift

increase

T =

Fd⊥

- F=force

- d⊥=distance away from line of action

What manipulations can you make to increase/decrease torque

- Longer lever arm = easier to lift something.

- longer resistance arm = resistance moves farther, but it's harder to move.

The same size torque can be created with a large force, small moment arm, or a

small force, large moment arm

What is the moment arm?

is the perpendicular distance between muscle's line pull and joint center

Largest moment arm at an angle of pull ~

900

Units for torque

newton meters (Nm)

What's the relationship between torque and angular acceleration

more torque you apply, the faster an object will rotate

It's more difficult to locate the COG in the ___ dimension due to the curvature of the spine

vertical

Lower COG for women due to a

larger pelvic girdle and narrower shoulders.

Infants and children have a higher COG due to

larger heads and shorter legs.

How to break down skills

1. Determine the Objective of the Skill

2. Note Special Characteristics of the Skill: Adjust to the environment

3. Study Top Performers

4. Divide the skill into Phases

5. Key Elements

6. Understand the Mechanical Reasons Behind Each Element

Laminar:

smooth and parallel (we want this flow!)

Low velocity relative to fluid medium

Turbulent:

mixing of adjacent fluid layers

High velocity relative to fluid medium

First class lever:

FAR (axis in middle)

First class lever Ex.

Seesaw, Scissors, neck extension

Second class lever:

FRA (resistance in middle)

Second class lever: Ex.

wheelbarrow, calf raise

Third-class lever:

RFA (force in middle)

Third-class lever: Ex.

knee extension, bicep curl

The most common lever systems in the body

3rd class levers

Mechanical Advantage =

Moment arm (force) / Moment arm (resistance)

The mechanical advantage is less than 1, which means

more force is required to move the load

What does air resistance do to objects?

- Slows objects down/Reduces speed

- Affects trajectory/Travel path

Right-hand rule and how it works with torque and moment arms

1. Point your fingers in the direction of the moment arm

2. Curl your fingers toward the direction of the force

3. Your thumb now points in the direction of the torque vector