Static Forces and Equilibrium

• Static forces definition covers forces acting on an object at rest or moving with constant velocity.
• In static forces, the forces are balanced, meaning the vector sum equals zero, so there is no acceleration.
• Common examples of static forces: gravitational, normal, tension, and friction in stationary systems.

Static Forces & Equilibrium

• Equilibrium happens when the net force and net torque on an object is zero.
• If the net force is zero, it is translational equilibrium, which means the object remains at rest or moves uniformly.
  • Mathematically shown as ΣFx = 0, ΣFy = 0, ΣFz = 0.
• When the sum of all torques about any axis is zero, that is rotational equilibrium.
  • This absence of torque prevents rotational acceleration.
  • Mathematically shown as Στ= 0.

Equilibrium in the Human Body

• Key factors affect equilibrium in the human body include:
• Center of Gravity (CoG)
• Base of Support (BoS)
• Line of Gravity
• Muscle Strength and Joint Stability

Center of Gravity (CoG)

• Point where the body's mass is evenly distributed.
• Lowering the CoG increases stability

Base of Support (BoS)

• Area beneath the body that supports its weight.
• A wider BoS enhances stability

Line of Gravity

• Imaginary vertical line extends from the CoG toward the ground.
• Stability is maximized when this line falls within the BoS.

Muscle Strength and Joint Stability

• Proper muscle engagement and joint positioning helps maintain equilibrium.
• Strong muscles generate needed force to counteract gravity and external forces while ensuring balance.
• Core muscles (abdominals, back, and pelvis) stabilize the spine and trunk to serve as a central foundation for balance.
• Lower limb muscles (quadriceps, hamstrings, calf muscles) support the body while standing or moving.
• Upper body muscles maintain posture and prevent imbalance, especially when lifting or carrying objects

Levers

• Simple mechanical device to lift or move a load with the help of an applied force.
• A lever is a rigid bar or beam that rotates around a fixed point called the fulcrum.
• Levers work based on mechanical advantage, so a small effort can move a larger load.
• Human body levers form with bones, muscles, and joints to produce movement.
• The three key components of levers include: Fulcrum, Effort, and Load
  • Fulcrum: The pivot point around which the lever rotates.
  • Effort: Force from muscles of the lever.
  • Load: Weight or resistance that the lever moves

First-Class Lever

• Fulcrum is located between the effort and load.
• First-class levers can increase force or speed, depending on relative distances.
• 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 effort.
• Second class levers always increase force but at the cost of speed and range of motion.
• The load (body weight) situated between the fulcrum (toes) and effort (calf muscles) describes a second-class lever.

Third-Class Lever

• Most common lever in the human body.
• Effort is exerted between the fulcrum and the load.
• Third-class levers increase speed and range of motion but require more effort.
• Example: elbow joint as fulcrum, biceps muscle to apply effort, holding hand as the load.

Elasticity

• Ability of a material or object to deform under strees and return to original state.
• Elasticity is quantified by parameters like the modulus of elasticity (Young's modulus).

Elastic properties of Biomaterials

• Understanding elasticity is crucial in materials science, engineering, and biomechanis. Biomaterial science is multidisciplinary so medical students should know it.
• Biomaterial science includes medicine, biology, chemistry, tissue engineering, and materials science.
• A biomaterial is any matter or surface that interacts with biological systems.

Physical Properties of Biomaterials

• Allow understanding the regularities between mechanical composition of materials
• Allow modeling the physical properties of materials
• Ceramics have very high strength and elastic modulus
• Metals have high strength and elastic modulus
• Polymers have low strength and elastic modulus

Elasticity and Hooke's Law

• Elasticity is a measure of a substance's ability to deform under stress and return to its original shape
• An ideal spring has an equilibrium length
• Hooke's Law describes the restoring force exerted by a spring.
  • It is proportional to the displacement from its equilibrium length.
  • Expressed mathematically as F = -kx, where:
  • F is force in Newtons
  • k is the constant (N/m)
  • x is amount of extension (m)
• When dealing with connecting identical springs:
  • Series ( with equal K): Length of the spring (L) directly proportional to AL (total length)
  • Paralell (with equal K): AL (length) directly proportional to 1/ Total number of springs

Young's Modulus

• E is the designation and formula is E = (F/A) / (ΔL/L)
  • F (Force) directly proportional to L (length)
  • (AL) (length) is inversely proportional to 1/A (cross-sectional area)
• Is a measure of how easily a material can stretch and deform

Elements of Theory of Elasticity

• Ultimate Stress: maximal load
• Yield Stress: yield load
• Elastic Region: stiffness
• Failure Point Stress is a function of deformation and yield

Mechanical Properties & Deformations of Bones

• Bone undergoes mechanical deformations that depend on the type of load:
  • unloaded
  • compression
  • tension
  • bending
  • shear
  • torsion
• Bone properties are age, gender, location in body, temperature, mineral and water content, and disease
• Decrease in bone mass is osteoporosis, which is due to age

Modulus Breakdown in Bones

• Bone is collagen fibers and an inorganic matrix
• Bones get less dense and more brittle with age, affecting their resistance to stress and fracture.

Chart Factors

• Longitudinal direction = along the length, along the bone's axis
  • When forces are applied along this, they function as compression or tension forces
• Transverse direction = perpendicular to longitudinal
  • When forces are applied here, the bone undergoes bending or shear forces