Phy Elasticity

• Static forces are forces acting on an object at rest or moving at a constant velocity.
• When the vector sum of forces equals zero, the forces are balanced and do not cause acceleration.
• Examples of static forces: gravitational force, normal force, tension, and friction in stationary systems.

Equilibrium

• Equilibrium happens when the net force and net torque on an object are zero. Types of equilibrium:
• Translational equilibrium means the sum of all forces is zero, keeping an object at rest or moving uniformly.
  • ΣFx = 0, ΣFy = 0, ΣFz = 0
• Rotational equilibrium means the sum of all torques is zero, preventing rotational acceleration.
  • Στ= 0

Factors Affecting Equilibrium

• Center of Gravity (CoG) is the point where the body's mass is evenly distributed; lowering the CoG increases stability.
• Base of Support (BoS) is the area beneath the body that supports its weight; a wider BoS enhances stability.
• Line of Gravity is an imaginary vertical line extending from the CoG toward the ground; stability is highest when this line falls within the BoS.
• Proper muscle engagement and joint positioning help maintain equilibrium.
• Strong muscles counteract gravity and external forces to maintain balance.
• Core muscles (abdominals, back, and pelvis) stabilize the spine and trunk for balance.
• Lower limb muscles (quadriceps, hamstrings, calf muscles) support the body while standing or moving.
• Upper body muscles help maintain posture and prevent imbalance, especially during lifting.

Levers

• A lever is a simple mechanical device that lifts or moves a load using an applied force.
• Levers consist of a rigid bar or beam rotating around a fixed point, known as the fulcrum.
• Levers work on the principle of mechanical advantage.
• In the body, levers are formed by bones, muscles, and joints, working together to produce movement. A lever includes three key components:
• Fulcrum (Pivot): The fixed point around which the lever rotates, usually a joint.
• Effort (Force): The force applied by muscles to move the lever.
• Load (Resistance): The weight or resistance that the lever moves.

First-Class Lever

• (Fulcrum is located between the effort and the load)
• First-class levers can either increase force or speed depending on the relative distances between the fulcrum, effort, and load.
• An example of a first-class lever in the human body is the neck during nodding.

Second-Class Lever

• (Load is located between the fulcrum and the effort.)
• Second-class levers always increase force but reduces speed and range of motion.
• An example of a second-class lever is standing on tip-toes because the load (body weight) is located between the fulcrum (toes) and the effort (calf muscles).

Third-Class Lever

• (Effort is applied between the fulcrum and the load).
• Third-class levers increase speed and range of motion, but require more effort.
• An example in the human body: holding a weight in hand during a bicep curl. The fulcrum is the elbow joint, the effort comes from the bicep muscle and the load is weight in the hand.

Elasticity

• Elasticity is a material or object's ability to deform under stress and return to its original shape when stress is removed.
• Elasticity is quantified by the modulus of elasticity or Young's modulus, describing a material's resistance to deformation.

Biomaterials

• A biomaterial is any matter or surface that interacts with biological systems. Biomaterial science is multidisciplinary:
• Medicine
• Biology
• Chemistry
• Tissue engineering
• Material science

Why Medical Students Should Know About Biomaterials:

• Biomaterials knowledge allows understanding the regularities between the mechanical properties and composition of biomaterials.
• Biomaterials knowledge allows modeling the physical properties of materials. Categories of biomaterials:
• Metals
• Composites
• Nanomaterials
• Biodegradable materials
• Passive surface coatings
• Materials derived from tissues
• Natural and synthetic polymers
• Biologically derived macromolecules
• Bioactive and tissue adhesive materials

Elastic Modulus

• Elastic modulus is high for ceramics and metals, but low for polymers.

Material Classification

• Homogeneous materials have the same composition and properties throughout; physical and chemical characteristics are uniform.
• Non-homogeneous materials vary in composition, structure, or properties across regions.

Isotropic Materials

• Isotropic materials have identical properties in all directions.

Orthotropic Materials

• Orthotropic materials are anisotropic with three mutually perpendicular axes of symmetry and different properties along each axis.

Hooke's Law

• Hooke's Law is the force needed to extend or compress a spring by some distance is proportional to that distance (F = -kx).
• An ideal spring has an equilibrium length.
• F (Force) ∝ ΔL (length)
• When two identical springs are in series, the length of the spring (L) ∝ ΔL (length).
• When two identical springs are in parallel, ΔL (length) ∝ 1/ total number of SPRINGS
• (ΔL) ∝ 1/A (cross-sectional area)

Young’s Modulus

• AL ∝ F * L/A
• F/A= E * ΔL/L
• F/A = STRESS (N/m²)
• ΔL/L= STRAIN (unitless)
• E = Young’s Modulus (N/m²)
• The Young's modulus (E) is a property of the material that tells us how easily it can stretch and deform.
• E = F/A / ΔL/L = STRESS (σ) / STRAIN (ε)
• Mechanical Deformations of Bones: bones can experience different types of mechanical deformations based on the applied load:
  • Unloaded
  • Compression
  • Tension
  • Bending
  • Shear
  • Torsion

Mechanical Properties of Bone:

• Properties are influenced by age, gender, location in the body, temperature, mineral content, water amount, and diseases like osteoporosis.
• As humans age, bones become less dense.
• Bone properties can change due to age-related factors like osteoporosis or changes in tissue composition. Bones become more brittle and less dense with age.
• Bone consists of collagen fibers and an inorganic matrix and can be examined as a fiber composite.
• Longitudinal Direction: The orientation along the length of the bone in compression or tension.
• Transverse Direction: Orientation perpendicular to the longitudinal axis and bending or shear forces act in this direction.
• Bone mineral behaves following Hookean elasticity.
• Collagen is a polymer displaying a J-shaped stress-strain curve.