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Understanding Newton's Laws in Biomechanics

May 5, 2025

Lecture on Newton's Laws and Biomechanics

Overview

  • Review of Newton's laws of motion in the context of biomechanics and human movement.

Newton's First Law

  • Inertia:
    • Objects resist changes in motion.
    • Related to mass: greater mass = greater inertia.
    • Requires an external force to change state of motion.

Newton's Second Law

  • Law of Acceleration:

    • Change in motion proportional to force impressed.
    • Formula: Force = Mass x Acceleration.
    • Acceleration depends on force magnitude and object mass.
    • Example: Baseball vs. shot put - less mass accelerates more.
  • Impulse:

    • Product of force and time duration.
    • Important for momentum changes in physical activities.
    • Graphical Representation: Force on y-axis, time on x-axis. Area under curve = impulse.
    • Longer force application = greater impact; shorter time = greater force needed.
  • Momentum:

    • Momentum = Mass x Velocity.
    • Impulse changes momentum (generally changes velocity).
    • Example: Football lineman vs. toddler – mass and velocity affect momentum.
    • Conservation of momentum in collisions - consistent unless external forces act.

Types of Collisions

  • Elastic Collisions:

    • Momentum completely transferred, objects bounce off.
    • Perfectly Elastic: Equal masses, velocities reversed post-collision.
    • Coefficient of Restitution: Measures elasticity; 1.0 for perfectly elastic, 0.0 for inelastic.
  • Inelastic Collisions:

    • Objects stick and move together post-impact.
    • Momentum conserved, velocity shared.

Practical Applications

  • Impulse-Momentum in Sports:

    • Catching ball: Soft techniques increase time, reduce peak force impact.
    • Vertical jumps: Optimize force and time for effective jump height.
  • Landing Mechanics:

    • Soft landings reduce mechanical load via increased contact time, lowering injury risk.

Newton's Third Law

  • Action-Reaction Principle:

    • Forces occur in pairs, equal in magnitude, opposite in direction.
  • Movement Applications:

    • Reaction forces essential in walking, jumping, throwing.

Friction

  • Role in Movement:

    • Acts parallel to contacting surfaces; crucial for stability and performance.
    • Coefficient of Friction: Determines force needed to overcome friction between surfaces.
  • Example of Friction:

    • Pulling a sled with considerations for frictional forces and angles.
    • Static vs. Kinetic Friction - differences in overcoming initial movement vs. maintaining motion.