8 Fluids

Describe the properties of a fluid.

• Distinguishing properties of solids, liquids, and gases stem from the varying interactions between atoms and molecules.
• A fluid is a substance that has no fixed shape.
• Fluids can be characterized by their density. Density is defined as a ratio of mass to volume. Relevant equation:
• An ideal fluid is incompressible and has no viscosity. TOPIC 8.1 Internal Structure and Density

Describe the pressure exerted on a surface by a given force.

• Pressure is defined as the magnitude of the perpendicular force component exerted per unit area over a given surface area, as described by the equation
• Pressure is a scalar quantity.
• The volume and density of a given amount of an incompressible fluid is constant regardless of the pressure exerted on that fluid.

Describe the pressure exerted by a fluid.

• The pressure exerted by a fluid is the result of the entirety of the interactions between the fluid’s constituent particles and the surface with which those particles interact.
• The absolute pressure of a fluid at a given point is equal to the sum of a reference pressure P0, such as the atmospheric pressure Patm, and the gauge pressure Pgauge. Relevant equation:
• The gauge pressure of a vertical column of fluid is described by the equation TOPIC 8.2 Pressure

Describe the conditions under which a fluid’s velocity changes.

• Newton’s laws can be used to describe the motion of particles within a fluid.
• The macroscopic behavior of a fluid is a result of the internal interactions between the fluid’s constituent particles and external forces exerted on the fluid.

Describe the buoyant force exerted on an object interacting with a fluid.

• The buoyant force is a net upward force exerted on an object by a fluid.
• The buoyant force exerted on an object by a fluid is a result of the collective forces exerted on the object by the particles making up the fluid.
• The magnitude of the buoyant force exerted on an object by a fluid is equivalent to the weight of the fluid displaced by the object. Relevant equation: TOPIC 8.3 Fluids and Newton’s Laws

Describe the flow of an incompressible fluid through a cross-sectional area by using mass conservation.

• A difference in pressure between two locations causes a fluid to flow.
  • i. The rate at which matter enters a fluid-filled tube open at both ends must equal the rate at which matter exits the tube.
  • ii. The rate at which matter flows into a location is proportional to the crosssectional area of the flow and the speed at which the fluid flows. Derived equation: V =Avt
• The continuity equation for fluid flow describes conservation of mass flow rate in incompressible fluids. Relevant equation: Av11 =Av22 TOPIC 8.4 Fluids and Conservation Laws

Describe the flow of a fluid as a result of a difference in energy between two locations within the fluid– Earth system.

• A difference in gravitational potential energies between two locations in a fluid will result in a difference in kinetic energy and pressure between those two locations that is described by conservation laws.
• Bernoulli’s equation describes the conservation of mechanical energy in fluid flow. Relevant equation:
• Torricelli’s theorem relates the speed of a fluid exiting an opening to the difference in height between the opening and the top surface of the fluid and can be derived from conservation of energy principles. Derived equation: BOUNDARY STATEMENT All fluids will be assumed to be ideal, and all pipes are assumed to be completely filled by the fluid, unless otherwise stated. AP Physics 1: Algebra-Based Course and Exam Description Laboratory Investigations AP PHYSICS 1