Linear Kinetics: Review & Recap

Characteristics of a Force

  • Definition: Forces are vectors, which means they have both magnitude and direction.

  • Unit of Measurement: Forces are measured in Newtons (N), mathematically defined as: (1N = kg⋅m/s2)

  • Representation: Typically, forces are represented in terms of body weight (BW).

Forces Acting on the Body

  • Activity Assessment: Understanding the relative force (N/BW) for different activities:

    • Walking: 1.0 – 1.5 N/BW

    • Running: 2.0 – 3.5 N/BW

    • Countermovement Jump: 2.2 N/BW

    • Triple Jump: 7.0 – 12.0 N/BW

Newton’s Laws of Motion

  1. 1st Law (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force.

  2. 2nd Law (Law of Acceleration): The acceleration of an object depends on the mass of the object and the amount of force applied, mathematically expressed as:
    F = ma

  3. 3rd Law (Law of Action-Reaction): For every action, there is an equal and opposite reaction.

Types of Forces

  • Contact Forces: Result from the interaction between two objects.

    1. Ground Reaction Force (GRF)

    2. Joint Reaction Force

    3. Friction

    4. Fluid Resistance

    5. Inertial Force

    6. Muscle Force

    7. Elastic Force

Force Platform

  • Ground Reaction Force (GRF): Components include:

    • Fx: Antero-posterior force

    • Fy: Vertical force

    • Fz: Medio-lateral force

  • Effects of Force:

    • Instantaneous Effect: The effect of force applied at an instant in time.

    • Cumulative Effect: The effect of force applied over time.

    • Displacement Effect: The effect of force applied over a distance, expressed as:
      F_t = m rac{Δv}{t}

Practical Application Example

  • Scenario: A person performs a vertical jump (SVJ) on a force plate.

  • Given Data:

    • Mass of the person = 65 kg

    • Measured Impulse = 125 N·s

  • Calculation Requirement: Calculate the person’s take-off velocity based on impulse and mass.