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Definition of Fluids: Both gases and liquids are classified as fluids because they are free to flow.

Solids:
- Characteristics: Rigidly bound molecules, definite shape, and volume (e.g., a phone, a coffee cup).
Liquids:
- Characteristics: Molecules are not tightly bound; maintain a definite volume but adapt to the shape of their container. Example: pouring coffee into a cup; it takes the cup's shape but stays within the same volume.
Gases:
- Characteristics: Molecules are not bound and have neither definite shape nor volume. Gases fill their container completely. Example: when a coffee cup is empty, it still contains air, demonstrating that gases occupy space.

Solids: When a force is applied to a solid, it transmits in the same direction throughout the solid.
Fluids: Fluids transmit forces equally in all directions; pressure at any point is the same in all directions. If a fluid is exerting force on a container, this force is perpendicular to the surface area.

Hydrostatic Pressure: The pressure exerted by a fluid at rest, increasing with depth due to the weight of the fluid above.
- Formula: $\Delta p = \rho g h$ where:
  - (\Delta p) = difference in pressure,
  - (\rho) = fluid density,
  - (g) = acceleration due to gravity,
  - (h) = height of fluid column.
Pressure Measurement: Often measured in millimeters of mercury (Torr).
- 1 Torr: Pressure exerted by a column of mercury 1 mm high.
- Other Units: Pascal (Pa) = Newtons/meter², with numerous relationships between units of pressure.

Definition: When a force is applied to a fluid under pressure, the pressure increase is transmitted undiminished throughout the entire fluid.
Analogy: Similar to stress in solids but referred to as pressure in liquids.
Application: Used in hydraulic systems; the output force can be calculated based on the areas and applied pressure.

Buoyant Force: The upward force exerted on an object immersed in a fluid, equal to the weight of the fluid displaced by the object.
Archimedes' Principle: States this relationship:
- $Fb = -\rho g Vd$ where:
 - (F_b) = buoyant force,
 - (\rho) = fluid density,
 - (g) = gravitational acceleration,
 - (V_d) = volume of fluid displaced.
Example: Ice cube floating in water displaces water, causing the water level to rise.

Definition: Molecules at the surface of a liquid experience different forces compared to those in the bulk, leading to cohesion at the surface.
Adhesion vs Cohesion: Adhesive forces can cause liquids to cling to container walls, while cohesive forces cause the fluid to hold together.
Contact Angle: The angle at which a liquid interface meets a solid surface; indicative of the balance between adhesive and cohesive forces.

Definition: Molecules that reduce the surface tension of a fluid, containing both hydrophilic (water-loving) and hydrophobic (water-repelling) ends.
Application: Surfactants help dissolve oils and dirt, examples include soaps and detergents.

Definition: Describes the behavior of fluids in narrow tubes due to adhesion and cohesion.
Example: Blood being drawn up through a small tube at a clinic shows the effects of these forces.

Description: Describes the conserved energy in flowing fluids, where pressure, kinetic, and potential energy are balanced.
- Equation: $P + \frac{1}{2}\rho v^2 + \rho g h = constant$ where:
  - (P) = pressure,
  - (\rho) = fluid density,
  - (v) = velocity,
  - (g) = acceleration due to gravity,
  - (h) = height above reference level.

Statement: The mass flow rate must remain constant from one cross-section to another in a tube.
- Mathematical Representation: $A1 v1 = A2 v2$ where:
 - (A) = area,
 - (v) = velocity.
Conclusion: As the area decreases, the velocity must increase to maintain flow.

Viscosity: A measure of a fluid's resistance to flow. It increases with temperature.
Types of Flow:
1. Laminar Flow: Fluid flows in parallel layers, minimal disruption. Present in arteries supplying organs.
2. Turbulent Flow: Irregular fluctuations and mixing in the fluid caused by increased velocity or sharp turns. It is more complex than laminar flow.

Definition: Ratio of inertial to viscous forces, helping predict the nature of fluid flow (laminar vs turbulent).
- Critical Velocity: Transition from laminar to turbulent flow is determined by calculating Reynolds number.

Heart Function:
- Blood exits the heart through arteries and returns via veins.
- The right side pumps blood to the lungs while the left side pumps oxygenated blood to the body.
Blood Flow Characteristics:
- Laminar Flow: Quiet and regular in healthy vessels.
- Turbulent Flow: Can create noises known as "bruit," which is a sign of vascular abnormalities.
Blood Pressure Measurements:
- Systolic Pressure: Peak pressure during heart contraction.
- Diastolic Pressure: Pressure during heart relaxation.
Plaque Buildup:
- Affects blood flow by narrowing arteries, resulting in increased velocity and decreased pressure, leading to potential ischemic conditions (e.g., heart attack, stroke).
- Critical stenosis is defined as narrowing above 80%, resulting in significant flow reductions and increased risk for cardiovascular events.
Risk Factors for Atherosclerosis include hypertension, cholesterol levels, obesity, and smoking.

Understanding fluid dynamics is crucial for comprehending blood flow and cardiovascular health, as the principles of physics govern physiological processes in the body.
Future applications include enhancing techniques in ultrasound and treatment approaches for diseases like atherosclerosis.