Fluids

AP Phyics 1

Quantity Definition Unit Measuring Device Equation
Mass Amount of matter kg Balance N/A
Volume Space occupied m³ Graduated Cylinder
Density Matter per space kg/m³ Calculated or Hydrometer
Weight Force of Gravity Newtons Spring Scale

What is a Fluid?

A fluid is a substance that:

  • Has no fixed shape
  • Takes the shape of its container
  • Can flow

Includes both liquids and gases

States of Matter - What are fluids?

States of Matter Mass Volume Density Shape
Solid Constant Constant Constant Constant
Liquid Constant 'Constant' 'Constant' Variable
Gas Constant Variable Variable Variable

Fields of Study in Fluids

Fluid Statics

  • Study of fluids at rest

Fluid Dynamics

  • Study of fluids in motion

Fluid Statics

  • Study of fluids at rest
  • Determining the density of objects and fluids
  • Factors that affect the pressure exerted by a fluid at rest
  • Archimedes' Principle: Buoyancy and the Buoyant Force

Density

Density: The ratio of mass to volume

  • Units: kg/m³, g/cm³, g/mL
  • Intensive property (doesn't depend on amount)
  • Key characteristic of materials

Density

Equation

Units

AP Physics: kg/m³
Chemistry: g/mL or g/cm³

Common Values

  • Air: 1.2 kg/m³
  • Water: 1,000 kg/m³
  • Salt Water: 1,030 kg/m³
  • Honey: 1,420 kg/m³
  • Iron: 7,870 kg/m³

Floating vs. Sinking

Objects float when:

  • Density of object < Density of fluid

Objects sink when:

  • Density of object > Density of fluid

Ideal Fluids

For the AP exam, we consider ideal fluids:

  • Incompressible: Volume doesn't change under pressure
  • No viscosity: Flows without resistance

Real fluids like water are close to ideal in many situations.

Density Determination Lab

Data for different amounts of the same unknown material

Mass (kg) Volume (×10⁻⁴ m³)
1.2 1.7
2.0 2.9
3.3 4.7
4.1 5.8
5.4 7.7
How can you graph the data to determine the density of the material?




Density Comparison Ranking Task

Object Mass(kg) Volume(m³)
A 7.2 0.0019
B 5.5 0.0026
C 2.3 0.00034
D 6.1 0.0077
Rank the densities of each object from the largest to the smallest.

Density Comparison Ranking Task - Solution

Object Mass(kg) Volume(m³) Density (kg/m³) Rank
A 7.2 0.0019 3,789 2
B 5.5 0.0026 2,115 3
C 2.3 0.00034 6,765 1
D 6.1 0.0077 792 4

AP Workbook Unit 8.A & 8.B