Linear Kinetics

Introduction to Kinetics

  • Kinetics: Study of forces that cause or restrain motion.

  • Importance of defining force, categorizing types, and equivalent systems of force.

  • Recap of key dimensions in biomechanics: space, time, and mass.

    • Mass: Measure of quantity of matter, proportional to inertia (resistance to acceleration).

    • SI unit of mass: Kilogram (kg).

    • Distinction between mass and weight:

      • Mass: Constant, does not change.

      • Weight: Force of gravity acting on an object, varies with location.

Defining a Force

  • Force: Mechanical disturbance exerted upon a body, capable of accelerating or deforming it.

    • Newton's 3rd Law: Every force has an equal and opposite reaction force.

    • System of Interest: Forces analyzed based on the defined system.

Units of Force

  • SI unit: Newton (N), with the relation 1 N = 1 kg*m/s².

  • US unit: Pound (1 lb = 4.45 N).

  • Common misconceptions in using kg as a unit of force in exercise physiology.

  • Introduction of the kilipond (kp) as a force measurement equivalent to gravity acting on 1 kg mass.

Characteristics of a Force

  • Force as a vector: has both magnitude and direction.

  • Point of Application and Line of Action: Critical for understanding force effects.

  • Naming convention for forces:

    • F(exerting body → acted body) helps in clarity.

Defining the System of Interest

  • Definition of the system of interest influences which forces are included.

  • Forces within the defined system are internal and ignored.

  • External Forces: Must be considered in analyses, categorized as:

    • Internal Forces: Act within the defined system (e.g., trunk force on arm).

    • External Forces: Interact from outside (e.g., gravity, pushes/pulls).

Types of Forces

  • Non-Contact Forces: Act over a distance (e.g., gravity).

  • Contact Forces: Act when bodies are touching (e.g., friction).

Non-Contact Forces

  • Only non-contact force considered is gravity:

    • F(gravity→object) = m*object * -9.8 m/s².

Contact Forces

  • Types of contact forces:

    • Normal Force: Component acting perpendicular to surfaces.

    • Frictional Force: Opposes sliding, can be static (not sliding) or dynamic (sliding).

    • Maximum Potential Friction vs. Actual Friction:

      • Maximum potential friction depends on normal force and surface interactions.

Concepts Related to Force

Pressure

  • Pressure: Force distributed over an area, crucial in deformable bodies.

Torques

  • Torque: Rotational effect of an applied force; calculated as Torque = F*d⊥.

Kinetics Problem-Solving Approaches

  1. Equilibrium Approach: Analyzing forces at a specific moment in time using Newton’s second law.

  2. Impulse-Momentum Approach: Forces acting over time to influence momentum.

  3. Work-Energy Approach: Relationship between work done and change in energy.

Equivalent Systems of Force

  • Use of equivalent systems simplifies complex force models:

    • Center of Gravity: Representing distributed gravitational force as a single force at the center of mass.

    • Center of Pressure: Simplifying contact force models to a point of application.

    • Force Couples: Two equal and opposite forces creating rotational effects.

Conclusion

  • Next steps include representation of forces using Free Body Diagrams (FBDs).

  • Importance of understanding Newton's three laws of motion to apply to rigid body mechanics.