KNES 361 L10

Lecture 10: Torque and Center of Gravity

Introduction to Torque and Center of Gravity

  • Discussion on the importance of center of gravity in movement and stability.

  • Understanding balance and stability through the identification of the center of gravity.

  • Overview of three main parts for this section:

    • Definition of center of gravity.

    • Connection between center of gravity, torque, and stability.

    • Explanation of line of gravity and its visualization.

Gravity and its Effects on the Body

  • Gravity pulls on every particle within the body's mass.

  • Instead of representing the effect of gravity with multiple force arrows:

    • A single weight vector is used to illustrate the overall gravitational force on the body.

  • The center of gravity (CG) is where this single weight vector is concentrated.

    • Acts as a point where all gravitational effects are balanced.

  • If an object is supported at its center of gravity, it remains balanced.

    • In uniform gravitational conditions (such as on Earth), CG is analogous to the center of mass.

    • The term centroid is also used for uniformly dense objects.

Characteristics of Center of Gravity

  • Symmetrical Objects: The center of gravity is positioned at the geometric center.

  • Irregular Shapes: The center of gravity shifts towards areas of greater mass concentration.

    • Example: An object that is not symmetric will have its CG closer to the denser mass side.

    • Human body posture impacts CG location due to irregular mass distribution.

Examples of Center of Gravity in Humans

  • In a relaxed, upright posture, the center of gravity is generally around the belly button.

  • Actions that alter upright posture (like raising arms, leaning, or squatting) shift a person's center of gravity due to changes in mass distribution.

    • Example: When leaning forward, CG can extend outside the body.

Influence of Posture on Center of Gravity and Stability

  • Comparison of center of gravity in various postures:

    1. Standing on One Foot:

    • Higher center of gravity → reduced stability.

    • Body control requires stiffness to maintain balance.

    1. Standing on Two Feet:

    • Increased stability with lower center of gravity.

    1. Sitting:

    • Lower center of gravity further increases stability.

  • Key takeaway: A lower center of gravity contributes to increased stability—important in sports (e.g., athletes adopting low stances).

Torque and Center of Gravity Connection

  • Introduction of the term low man wins in football to illustrate the importance of posture on stability and torque.

  • Understanding why low posture enhances stability:

    • Lower center of mass increases stability and makes it harder for opponents to apply torque, or the twisting effect, on the player.

    • Increasing momentum through rising can lead to greater torque, demonstrating how posture impacts these forces.

Line of Gravity

  • Definition of line of gravity:

    • An imaginary vertical line extending from the center of gravity straight down to the ground.

  • Visual representation illustrates the concept:

    • The center of gravity is around the belly button, and the red dotted line represents the line of gravity.

  • Significance of line of gravity:

    • Indicates the direction in which gravity acts.

    • Essential for stability analysis as it reveals where this line intersects with support.

Conclusion

  • Recap of the interrelationship between torque, center of gravity, and line of gravity.

  • Emphasis on understanding these concepts for practical applications in activities like sports and daily movements.