Subdivisions of Mechanics

Mechanics is such a highly developed physical science that many subdivisions have arisen. In a typical physics or mechanical engineering curriculum, an entire series of courses covers the subspecialties of mechanics. Understanding these subdivisions helps clarify terminology used in the speech science literature and identifies which mechanical principles apply to specific voice production problems.

Dynamics Versus Statics

Dynamics

If objects under investigation are being accelerated, the study falls into a branch of mechanics called dynamics. This is the general case where:

• Forces are not in equilibrium
• Objects change velocity (speed or direction)
• Acceleration occurs
• Newton’s second law (f = ma) applies directly

Applications in voice science:

• Vocal fold tissue during vibration
• Air particles accelerating through the glottis
• Rapid jaw movements during articulation

Statics

In some special situations, all forces are in equilibrium and the object may actually be at rest or moving only in a straight line at a constant velocity. The study then falls into the realm of statics.

Characteristics:

• Sum of all forces equals zero
• No acceleration occurs
• Object is either at rest or moving at constant velocity in straight line
• Special case of dynamics

Why it’s important: Statics is merely a component of dynamics, but its application in structural design (bridges, buildings, dams) is so prevalent that it has become a field of study in its own right.

Applications in voice science:

• Sustained postures of articulators
• Static lung volumes
• Equilibrium positions of laryngeal cartilages

Kinematics Versus Kinetics

Kinematics

Kinematics deals with movement only, without specific reference to the forces that cause the movement.

Focus:

• Describing trajectories
• Measuring velocities
• Calculating accelerations
• Analyzing motion patterns

Applications in voice science:

• Tracking vocal fold displacement patterns
• Measuring articulatory movements
• Analyzing jaw trajectories during speech
• Describing mucosal wave propagation

Kinetics

Kinetics deals with movement as a consequence of known or assumed forces.

Focus:

• Relating forces to resulting motion
• Applying Newton’s second law
• Predicting motion from forces
• Inferring forces from observed motion

Challenge in voice science: Kinetic studies in phonation and articulation have been rare because the forces of interest are internal muscular forces, which are difficult to define and measure because of:

• Multiple attachments
• Complex muscle orientations
• Difficulty determining where one part of the body ends and another begins

Continuum Versus Particle Mechanics

Particle Mechanics

Particle mechanics deals with matter located at discrete points, like a bullet flying through the air.

Assumptions:

• Object treated as point mass
• All mass concentrated at single location
• Dimensions negligible compared to distances traveled

Example: Projectile motion (bullet, baseball)

Continuum Mechanics

Continuum mechanics deals with matter that appears to be distributed over some region of space, like clouds or bodies of water.

Characteristics:

• Mass distributed continuously through space
• Deformation of entire region considered
• Internal structure affects behavior

Further subdivision:

• Fluid mechanics: Liquids and gases
• Solid mechanics: Rigid bodies

Importance: Fluid and solid mechanics are the basic building blocks of laryngeal biomechanics. They will be applied in this chapter and in Chapter 3 on respiration.

Fluid Versus Solid Mechanics

Fluid Mechanics

The mechanics of liquids and gases. Fluids are substances that:

• Tend to flow in pipes or ducts
• Assume the shape of their container
• Cannot maintain shear stresses indefinitely

Applications in voice science:

• Airflow through vocal tract
• Aerodynamic forces on vocal folds
• Bernoulli effect
• Turbulent versus laminar flow

Solid Mechanics

The mechanics of rigid bodies. Solids are substances that:

• Maintain their own characteristic shape
• Can sustain shear stresses
• Deform but return to original shape (if elastic)

Applications in voice science:

• Vocal fold tissue deformation
• Cartilage framework behavior
• Elastic properties of lamina propria layers
• Ligament and tendon mechanics

Summary Table

Choosing the Right Approach

Different voice production phenomena require different mechanical approaches:

For vocal fold vibration:

• Dynamics (tissue is accelerating)
• Continuum mechanics (tissue is distributed)
• Solid mechanics (for tissue) + Fluid mechanics (for air)
• Kinetics (relating aerodynamic forces to motion)

For sustained vowel production:

• Statics (for articulatory posture)
• Fluid mechanics (for airflow)
• Kinematics (for describing steady articulator positions)

For consonant articulation:

• Dynamics (rapid movements)
• Kinematics (describing trajectories)
• Kinetics (forces producing movements)

Historical Context

The subdivisions of mechanics reflect both the historical development of the field and practical applications:

• Statics developed first for architecture and construction
• Dynamics emerged with study of planetary motion and projectiles
• Fluid mechanics advanced with hydraulics and aeronautics
• Continuum mechanics arose from need to describe deformable materials

Each subdivision brings its own mathematical tools and physical insights, all ultimately grounded in Newton’s laws.

Summary

Mechanics subdivides into specialized branches that address different aspects of motion and force. The major divisions—dynamics versus statics, kinematics versus kinetics, and continuum versus particle mechanics—reflect both the types of questions asked and the methods used to answer them.

For voice science, the most relevant subdivisions are:

• Dynamics: Because phonation involves rapid acceleration
• Kinetics: To relate forces to observed motion (though difficult due to inaccessible internal forces)
• Continuum mechanics: Because biological tissues are distributed matter
• Both fluid and solid mechanics: Because voice production involves interaction between air and tissue

Understanding these subdivisions helps in selecting appropriate analytical tools and recognizing which mechanical principles apply to specific problems in voice production.

Key Takeaways

• ✅ Dynamics studies accelerated objects; statics studies objects in equilibrium
• ✅ Kinematics describes motion; kinetics relates motion to forces
• ✅ Particle mechanics treats discrete masses; continuum mechanics treats distributed matter
• ✅ Fluid mechanics applies to liquids and gases; solid mechanics to rigid bodies
• ✅ Voice production requires both fluid mechanics (air) and solid mechanics (tissue)
• ✅ Kinetic studies are rare in phonation due to difficulty measuring internal muscular forces

Related Topics

• Definitions of Mechanical Quantities
• The Mechanical System
• Elements of Continuum Mechanics

Further Reading

1. Halliday, D., & Resnick, R. (1988). Fundamentals of physics. New York: Wiley.
2. Fung, Y. C. (1981). Biomechanics: Mechanical properties of living tissues. New York: Springer-Verlag.