Fluids are all around us, from the air we breathe to the water we drink. Understanding their properties is key to grasping how they behave in different situations. This topic dives into the phases of matter, density, pressure, and fluid dynamics.

We'll explore how molecules in fluids interact, why some fluids are compressible while others aren't, and how pressure affects fluid behavior. We'll also look at real-world applications like hydraulic systems and airplane wings, connecting theory to everyday life.

Phases of Matter and Fluid Properties

Phases of matter in fluids

• Solid has a rigid and fixed shape because its atoms or molecules are closely packed and have limited motion
• Liquid has a definite volume but no fixed shape since its atoms or molecules have more freedom to move than in solids
• Gas has no definite shape or volume as its atoms or molecules move randomly with large distances between them

Molecular structure and fluid behavior

• Fluids (liquids and gases) flow and deform under applied forces because their molecules have weaker intermolecular forces compared to solids, allowing them to move past one another
• Liquids have stronger intermolecular forces than gases, making them more difficult to compress and giving them a definite volume, while gases expand to fill their container
• Surface tension in liquids results from these intermolecular forces, causing the liquid surface to behave like an elastic membrane

Compressible vs incompressible fluids

• Compressible fluids (gases, such as air) have a density that changes significantly with pressure
• Incompressible fluids (liquids, such as water, and dense gases) have a density that remains nearly constant with changes in pressure

Density and Pressure

Density and its SI units

• Density ($\rho$) is the mass per unit volume of a substance, calculated using the formula $\rho = \frac{m}{V}$
• SI units for density are kilograms per cubic meter ($kg/m^3$)

Density comparisons in fluids

• Water has a density of 1000 $kg/m^3$
• Air at sea level has a density of 1.225 $kg/m^3$
• Oil has a density ranging from 800-900 $kg/m^3$
• Mercury has a density of 13,600 $kg/m^3$

Pressure and its SI units

• Pressure ($P$) is the force per unit area applied perpendicular to a surface, calculated using the formula $P = \frac{F}{A}$
• SI units for pressure are pascals ($Pa$) or newtons per square meter ($N/m^2$)
• Atmospheric pressure at sea level is 101,325 $Pa$ (1 atm)
• Gauge pressure is the pressure relative to atmospheric pressure
• Absolute pressure is the sum of gauge pressure and atmospheric pressure
• Hydrostatic pressure is the pressure exerted by a fluid at rest due to its weight

Pressure, force, and area relationship

• Pressure is directly proportional to force and inversely proportional to area
  • Increasing force on a constant area increases pressure
  • Increasing area with a constant force decreases pressure
• Applications include hydraulic lifts that use a small force over a small area to generate a large force over a large area, and sharp objects exerting more pressure than blunt objects for the same force due to their smaller area

Problem-solving with fluid pressure

1. Use the pressure formula ($P = \frac{F}{A}$) to calculate pressure, force, or area when given the other two quantities
2. Consider the effects of atmospheric pressure and gauge pressure when solving problems
3. Apply the concept of pressure to real-world situations (hydraulic systems, tire inflation, underwater depth)

Fluid Dynamics

Bernoulli's Principle

• Describes the relationship between fluid pressure, velocity, and height in a flowing fluid
• As fluid velocity increases, pressure decreases, and vice versa
• Explains lift in airplane wings and the curved flight of spinning balls

Viscosity and Fluid Flow

• Viscosity is a measure of a fluid's resistance to flow
• Higher viscosity fluids (like honey) flow more slowly than lower viscosity fluids (like water)
• Affects fluid behavior in pipes, blood flow in vessels, and lubrication in engines

Buoyancy

• The upward force exerted by a fluid on an immersed object
• Explains why objects float or sink in fluids
• Related to the volume of fluid displaced by the object and the densities of the object and fluid