Biomechanics Week 2: Introduction to Forces

Biomechanics Week 2: Introduction to Forces

• Concepts from Last Week
  • Mass: The amount of "stuff" an object contains, measured in kilograms (kg)
  • Weight: The amount of gravitational force exerted on an object, measured in Newtons (N)
  • Weight is calculated by multiplying mass by the acceleration due to gravity (9.8 m/s²)
  • Inertia: Resistance to action or change in acceleration; directly proportional to mass (more mass = more inertia)

Lesson Outline

• Explain what a system is and how it moves
• Discuss free body diagrams
• Introduce Newton's laws of motion
• Define key terms
• Compare internal and external forces
• Explore factors affecting friction
• Describe concurrent force and depict graphically
• Review static equilibrium

Describing Biomechanics

• System: Any structure or organization of related structures whose state of motion is of analytical interest (e.g., entire body, single joint, or object moved by the body)
• Anatomical Position: Standard reference position for directional terms
  • Directional terms: (superior/inferior, anterior/posterior, medial/lateral, proximal/distal, superficial/deep)
  • Planes of motion: (sagittal, frontal/coronal, transverse)
  • Axis of motion: (superoinferior, anteroposterior, mediolateral)

Describing Biomechanics (cont.)

• Center of Mass: The point at the intersection of the three cardinal planes representing the average location of a system's mass.
• Center of Gravity: synonymous with center of mass.

Spatial Frames of Reference

• Cartesian Coordinate System: A frame of reference defined by an origin and two or three orthogonal axes each passing through the origin and defining one spatial dimension.
• 2-D (x, y) or 3-D (x, y, z) coordinate systems.
• Can be global (fixed) or local (somatic).

Newton's Laws of Motion

• Law of Inertia:
  • Every object at rest will remain at rest unless a force acts upon it.
  • Every object in motion will continue in motion unless a force acts upon it.
• Law of Acceleration: Force = mass x acceleration (F = ma)
• Law of Action and Reaction: For every action, there is an equal and opposite reaction.

Free-Body Diagrams

• A simplified representation of a system free of the movement environment, often a stick figure or geometric model, showing the center of mass and points of contact with the environment.
• Used to illustrate all external forces acting on an object.

Force

• Direction: The path of applied force.
• Orientation: Alignment of the vector in relation to cardinal directions.
• Point of Application: Exact location of force.
• Magnitude: size or strength of the force.
• Line of Action: The imaginary line extending along the vector from the tip to the tail.

Vector

• Describes a force's magnitude and direction in graphical form.
• Length of the arrow = force's strength.
• Direction of the arrow = direction of the force.

Internal vs. External Forces

• Internal forces: act within the object or system being investigated.
• External forces: act on an object due to its interaction with the environment (applied, normal, contact forces, friction, air resistance).

Friction

• Exists between surfaces of two objects in contact that's moving in different directions.
• Opposes the motion between sliding surfaces.
• Two types of Friction
  • Static friction
  • Kinetic friction
• Factors affecting friction
  • Weight
  • Surface area
  • Contacting materials

Friction in Training and Exercise

Calculating Friction

Important Terms

• Resultant Force: The sum of two or more forces.
• Net Force: Positive and negative forces acting on an object.
• Concurrent Forces: Forces that do not act along the same line of action, but that act through the same point at the same time.
• Collinear Forces: Forces that have the same line of action, though sometimes in different directions.
• Static Equilibrium: Analysis of objects at rest or moving with constant velocity (zero acceleration).

Calculating Resultant Force

• Concurrent Forces: Forces that act in different planes of motion
  • Use Trigonometry (Right angle example) to solve for Magnitude and Direction. (Method Vector Resolution, Vector Chain Method)
  • Use methods like Pythagorean Theorem (Magnitude) and Vector Resolution/Chain Method (Direction)

Additional Practice Questions

Quiz

• Available date and time
• There are no extensions / no re-take

Coming Up

• Next Week's lecture: Linear Kinematics
• Upcoming Assessments: Quiz #2 (January 24-26), Quiz #3 (Jan. 30-February 2), and Literature Review #1 (Feb. 6).