Linear Kinetics of Human Movement - Newton's Laws of Motion

Linear Kinetics of Human Movement

Objective
  • Review Newton's three laws
  • Understand the application of Newton's laws to biomechanics
Newton's Three Laws of Motion

  1. First Law (Law of Inertia)

    • Every object in a state of uniform motion tends to remain in that state unless acted upon by an external force.
    • Inertia: Represents resistance to acceleration. The more mass an object has, the more inertia it has.
    • Example: An object in space continues to move indefinitely unless a force is applied.
    • Test your knowledge:
      • What happens to a screwdriver if an astronaut lets go of it in space?
      • How does the astronaut's situation differ from that on Earth?

  2. Second Law (Law of Acceleration)

    • The relationship between mass ($m$), acceleration ($a$), and force ($F$) is defined by the equation: F=maF = ma.
    • Acceleration is directly proportional to the force applied and inversely proportional to the body's mass.
    • Example: A heavier object accelerates less than a lighter object when the same force is applied.
    • Test your knowledge:
      • If the applied force is reduced by 50%, how does acceleration change?
      • What happens to velocity in this case?

  3. Third Law (Law of Reaction)

    • For every action, there is an equal and opposite reaction. When one body exerts a force on another, the second body exerts an equal and opposite force on the first.
    • Example: The weight of a box on a table generates an upward reaction force by the table.
    • Test your knowledge:
      • Ground reaction forces act in the same direction as the forces applied by your foot on the ground - true or false?
Detailed Examples of Newton's Laws
First Law Example: In Space
  • If an astronaut releases a screwdriver, it will float away in the direction it was released since no external force is acting on it to stop it.
Second Law Example: High Jumper
  • A high jumper exerts a force of 3extkN3 ext{kN} against the ground at takeoff. The ground exerts an equal and opposite force of 3extkN3 ext{kN} upwards on the jumper.
Third Law Example: Weight on Table
  • The weight of a box sitting on a table generates a downward force, and the table provides a reaction force upward that is equal in magnitude but opposite in direction.
Free Body Diagrams (FBD)
  • Definition: A sketch showing a defined system in isolation with all force vectors acting on it.
  • Purpose:
    • Pictorial representation of forces in action following Newton’s 2nd Law.
    • Displays all forces and torque vectors external to the system, including the line of action and point of application.
Review Questions
  • 1st Law: Is a bicyclist turning a corner at 15extkm/hr15 ext{ km/hr} in a state of uniform motion?
  • 2nd Law: If force is increased, what happens to acceleration?
  • 3rd Law: True or false: Ground reaction forces act in the same direction as the force your foot applies to the ground?
  • Free Body Diagrams: Which of Newton’s Laws are they associated with?