Linear Kinetics and Forces in Biomechanics

Lesson Objectives

  • Sporting Applications: Understanding the role of kinematics in sports performance.
  • Definition of Force: Recognizing force characteristics and types.
  • Three Laws of Motion: Exploring their impact on human movement and sports.
  • Contact vs Non-Contact Forces: Identifying forces impacting motion.
  • Free Body Diagrams: Utilizing diagrams in mechanical analysis.
  • Mechanical Analysis Process: Assessing forces at play during motion.

Linear Kinetics

  • Definition: A branch of mechanics focused on the causes of motion, known as kinetics.
  • Motion Initiators: Kinetics explains why movement occurs (e.g., runners leaning on curves, pitchers curving balls).
    • Reference: Hall. 9th Ed. Ch. 12, pp. 356-365.

Force

  • Definition: Any interaction (push/pull) between two objects causing acceleration, either positive or negative.
  • Motion Dynamics: Objects move when a force exceeds their inertia.
  • Force as a Vector:
    • Magnitude: Amount of force applied.
    • Direction: Critical in determining effect (e.g., pushing vs pulling).
  • Unit of Force: Newton (N).

Characteristics of Forces

  • Vectors: Represented by arrows; length indicates magnitude; direction shows the point of application.
  • Point of Application: The specific location where the force is applied (e.g., muscle attachment to bone).
  • Line of Application: Straight line indicating the direction of the force, extends infinitely.
  • Angle of Application: Orientation with respect to an X-Y coordinate system.

Resolution of Forces

  • Vector Nature: Forces can be resolved into horizontal and vertical components.
  • Net Effect: Combined effect of all acting forces.
  • Force Systems: Can be coplanar or concurrent, leading to resultant forces.
  • Resolution Techniques: Use of trigonometry (sine/cosine) to find components.

Laws of Motion

Newton's 1st Law - Law of Inertia

  • Definition: A body remains at rest or moves at constant velocity until acted on by a net external force.
  • Inertia: Resistance to change in motion relative to mass.
  • Example: To lift a 70 kg barbell, a force greater than 686.7 N is required.

Newton's 2nd Law - Law of Acceleration

  • Definition: A net force causes acceleration proportional to the force and inversely proportional to mass.
    • Formula: F = ma (Force = mass x acceleration).
  • Momentum: Can also be described via changes in momentum with time.

Newton's 3rd Law - Law of Reaction

  • Definition: For every action, there is an equal and opposite reaction.
  • Example: When a person jumps, they exert force on the ground, which pushes back with equal force.

Types of Forces

Non-contact Forces

  • Definition: Forces exerted without direct contact; gravity is a primary example.
  • Effects: Causes free-fall motion and celestial movements (e.g., moons, planets).
  • Gravitation Law: All bodies attract each other; F = Gm1m2/r².

Contact Forces

  • Definition: Forces exerted through direct contact (e.g., pushes/pulls between objects).
  • Types in Biomechanics:
    • Ground Reaction Force (GRF)
    • Joint Reaction Force (JRF)
    • Friction and Fluid Resistance

Ground Reaction Force (GRF)

  • Definition: The force exerted by the ground in reaction to the force applied by a body.
  • Components: Measured in three dimensions - vertical (Fz), medio-lateral (Fx), and antero-posterior (Fy).
  • Measurement: Conducted via force platforms; expressed relative to body weight.
  • Differences in Movement: GRF during walking versus running varies significantly, ranging from 1-1.2 BW for walking to 3-5 BW for running.

Joint Reaction Force (JRF)

  • Definition: The net force acting across a joint, reflecting internal forces.
  • Example: A standing thigh exerts a downward force across the knee, while the leg applies an equivalent upward force.

Friction

  • Definition: A force acting parallel to the interface of two surfaces, opposing motion.
  • Static and Kinetic Friction: Static (Fs) prevents movement, while kinetic (Fk) occurs during motion.
  • Determining Factors: Includes the coefficient of friction, normal reaction force, and surface nature.

Mechanical Analysis Process

Summation of Forces

  • Static Analysis: All forces in equilibrium (000203).
  • Dynamic Analysis: Forces lead to acceleration; requires careful measurement of all acting forces.

Free Body Diagrams (FBD)

  • Purpose: Visual representations to analyze forces acting on a system.
  • Usage: Essential for understanding complex interactions and calculations in biomechanics.

Final Notes

  • Importance of Mechanics in Sports: Understanding these principles can inform training and rehabilitation protocols, enhance performance, and prevent injuries. Apply concepts of force, friction, and motion to optimize movement in various sports settings.