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.