Unit 8: Fundamentals of Fluids

Fluid

Any substance that can flow and change the shape of the volume it occupies, includes liquids and gases.

Solid

A state of matter where atoms are arranged in fixed lattice structures, maintaining a definite shape and volume.

Liquid

A state of matter where atoms or molecules are close together but free to slide past one another, having a definite volume but no definite shape.

Gas

A state of matter where molecules are far apart and move randomly at high speeds, with neither definite shape nor volume.

Density (ρ)

Mass per unit volume, defined as ρ = m/V.

Incompressibility

The property of liquids in AP Physics 1, where density remains constant regardless of pressure.

Compressible

Describing gases which can have significantly changing densities due to volume changes.

Pressure (P)

The magnitude of force applied perpendicular to a surface, divided by the area of that surface.

Absolute Pressure (P_abs)

The true total pressure acting at a point, including atmospheric pressure.

Gauge Pressure (P_gauge)

Pressure difference between the fluid and the atmosphere, excluding atmospheric pressure.

Hydrostatic Pressure

The pressure in a static fluid, varying with depth based on the weight of the fluid column above.

Depth (h)

The distance below the surface of a fluid, affecting hydrostatic pressure.

Pressure and Depth Relationship

Pressure increases with depth in a fluid due to the weight of the fluid above.

Kinetic Theory of Fluids

Concept explaining that fluids consist of particles moving in random directions, exerting pressure when colliding with surfaces.

SI unit for pressure

Pascal (Pa), equivalent to 1 N/m².

Relationship between Gauge and Absolute Pressure

Pabs = Patm + P_gauge.

Hydrostatic Paradox

Pressure at a specific depth depends only on depth, fluid density, and gravity, not the container shape.

Fluid Equilibrium

When pressure within a static fluid is uniform at a given depth, affected only by local density and gravity.

Fluid Density of Water

Approximate density of freshwater commonly used, which is 1000 kg/m³.

Density of Air

Approximate density of air at sea level is 1.2 kg/m³.

Density of Mercury

Approximate density of mercury is 13,600 kg/m³.

Common Mistake: Depth vs. Altitude

In pressure equations, depth is measured downward from the fluid surface, not from the bottom of the container.

Common Mistake: Neglecting Atmospheric Pressure

Always add atmospheric pressure when calculating absolute pressure.

Common Mistake: Pressure Direction

Pressure is a scalar and does not have direction; however, the resultant force from pressure acts perpendicularly.