Free Body Diagrams for Human Movement Notes

Free Body Diagrams for Human Movement

• Objectives
  • Understand how free body diagrams are used to construct a model of the forces acting on a body.

Purpose of a Free Body Diagram

• A free body diagram is a pictorial representation of the left side of Newton’s 2nd Law.

Developing a Free Body Diagram

• Five Steps to Create a Free Body Diagram:
  1. Identify the system of interest.
  2. Redraw the system as a stick figure.
  3. Draw the weight force pointing down from the center of gravity (if applicable).
  4. Replace all external contacts with appropriate forces.
  5. Show the reference frame.

Replacing External Contacts with Appropriate Forces

• To do this:
  • Understand the mechanical actions of common forces in human movement.
  • Identify potential action forces between the system of interest and the outside world.
  • Provides a framework to evaluate forces.

Mechanical Actions of Common Forces in Human Movement

1. Rope-like Structures

   • Examples:
     • Holding onto rings in gymnastics.
     • Tug of war.
     • Muscle/tendon complex.
     • Ligaments.
   • Action:
     • Force pulls along the line of action of the structure.

2. Contact with a “Rough” Surface

   • Examples:
     • Ground reaction force.
     • Leaning against a wall.
   • Action:
     • Replace the surface with two forces: friction along the surface and a normal force perpendicular to the surface, both acting at the center of pressure.

3. Contact with a Smooth Surface

   • Example:
     • Ground reaction force when standing on ice.
   • Action:
     • One perpendicular force (normal force) acting at the center of pressure.

4. Free Rotation Around an Axle

   • Example:
     • Bone-on-bone forces.
   • Action:
     • Force in any direction acting at the center of rotation.

5. Constrained Rotation Around an Axle

   • Example:
     • Grasping a tool with an offset center of gravity.
   • Action:
     • Both a force and a torque are represented; the torque can be either clockwise or counter-clockwise.

Creating a Free Body Diagram for a Gymnast

• Step One: Identify system of interest
  • Focus is on determining ground reaction forces of the gymnast.
• Step Two: Redraw as a stick figure.
• Step Three: Draw weight force (Fg) if applicable.
• Step Four: Replace external contacts with appropriate forces:
  • Fg (Weight force),
  • FGRF (Ground Reaction Force - normal),
  • Ffriction (Frictional force).
• Step Five: Add reference frame indicators.

Recap of Steps to Create a Free Body Diagram

• 5 steps to creating a free body diagram.
• Understanding common forces in analyzing human movement.

Next Class Topics

• Define friction and understand major determinants of friction.
• Understand ground reaction force and its relationship to friction.
• Examples of how friction and ground reaction forces contribute to movement.
• Discuss static equilibrium.