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Unit 1 Overview: Fluids

10 min readjanuary 29, 2023

Daniella Garcia-Loos

Daniella Garcia-Loos


AP Physics 2 🧲

61 resources
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Fluids is a branch of physics that studies the properties and behavior of liquids and gases.
At an AP Physics 2 level, key concepts and formulas that students should know include:
  1. Density: The mass per unit volume of a fluid, typically measured in kg/m^3. Density is an important property of fluids, as it affects how they behave in different situations.
  2. Pressure: The force per unit area exerted by a fluid on an object. Pressure is often measured in Pascals (Pa) or atmospheres (atm). Pressure can be calculated using the formula P = F/A, where P is pressure, F is force, and A is area.
  3. Buoyancy: The upward force exerted on an object submerged in a fluid, caused by the fluid's pressure. Buoyancy is determined by the fluid's density and the volume of the submerged object.
  4. Archimedes' Principle: The principle that states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object. This principle can be used to calculate the buoyancy of an object in a fluid, and to determine whether an object will float or sink.
  5. Continuity Equation: The equation that states that the product of the fluid's velocity and cross-sectional area is constant for a given point in a fluid flow. This equation can be used to calculate the velocity of a fluid at different points in a flow.
  6. Bernoulli's Equation: The equation that relates the pressure, velocity, and height of a fluid in a steady flow. This equation can be used to calculate the pressure, velocity, and height of a fluid at different points in a flow, and to understand how fluid flow affects pressure and velocity.
  7. Viscosity: The property of a fluid that causes it to resist flow. Viscosity can be affected by temperature, pressure, and the properties of the fluid itself.

1.1 Fluid Systems

Fluid systems are systems that involve the movement and behavior of fluids, such as liquids and gases. At an AP Physics 2 level, students should understand the different types of fluid systems and the principles that govern their behavior.
One important type of fluid system is the open system, which involves the exchange of matter or energy with the surroundings. An example of an open system is a pipe that carries water from a reservoir to a city. In this system, water is flowing through the pipe and entering and leaving the system as it is consumed by the city.
Another important type of fluid system is the closed system, which does not involve the exchange of matter or energy with the surroundings. An example of a closed system is a sealed container filled with a gas. In this system, the gas is confined within the container and its behavior is determined by the properties of the gas and the container itself.
One of the key principles that govern fluid systems is the conservation of mass. This principle states that the total mass of a closed system remains constant over time, regardless of the changes in temperature, pressure, or other properties of the fluid.
Another important principle that governs fluid systems is the conservation of energy. This principle states that the total energy of a closed system remains constant over time, regardless of the changes in temperature, pressure, or other properties of the fluid.

1.2 Density

Density is a measure of how much mass is contained in a given volume of a substance. At an AP Physics 2 level, students should understand the concept of density and how it is related to other physical properties of a substance.
The density of a substance is typically represented by the Greek letter "rho" (ρ) and is defined as the mass of the substance divided by its volume. The unit of density is typically kilograms per cubic meter (kg/m^3) in the International System of Units (SI).
The density of a substance can be determined by measuring its mass and volume and then using the formula:
density (ρ) = mass (m) / volume (V)
It is important to note that density is an intensive property, meaning that it does not depend on the size of the sample. For example, the density of a substance will be the same whether it is in a small container or a large container, as long as the mass and volume of the sample are measured correctly.
Another important concept related to density is that of buoyancy, which is the force that opposes an object's weight in a fluid. Buoyancy is determined by the density difference between the object and the fluid. An object that has a lower density than the fluid it is in will float, while an object that has a higher density than the fluid will sink.
Density can also be used to determine whether a substance is a solid, liquid or gas. The densities of solids are generally higher than that of liquids, while the densities of liquids are higher than that of gases. This is due to the fact that the particles in solids are closely packed together, while the particles in liquids and gases are farther apart.
Finally, the density of a substance can be affected by changes in temperature and pressure. As temperature increases, the density of a substance will generally decrease, while as pressure increases, the density of a substance will generally increase.

1.3 Fluids: Pressure and Forces

Pressure is a measure of the force exerted per unit area on a surface. The unit of pressure in the International System of Units (SI) is the pascal (Pa), which is equivalent to one newton per square meter (N/m^2). Pressure can also be measured in other units such as atmospheres (atm) or pounds per square inch (psi).
One important concept related to pressure is that of fluid statics, which is the study of fluids at rest. In fluid statics, the pressure at any point within a fluid is the same in all directions, and is called the hydrostatic pressure. This principle is known as Pascal's principle, which states that the pressure at any point in a fluid is transmitted undiminished in all directions.
Another important concept related to pressure is that of fluid dynamics, which is the study of fluids in motion. In fluid dynamics, the pressure at any point within a fluid can vary depending on the direction and magnitude of the fluid flow. This principle is known as Bernoulli's principle, which states that an increase in the velocity of a fluid is accompanied by a decrease in pressure, and vice versa. This principle is often used to understand the behavior of fluids in situations such as pipe flow and wing design.
When it comes to fluid forces, the weight force due to gravity acting on the fluid is known as the fluid weight force. The fluid weight force is the result of the weight of the fluid and acts in the downward direction.
Another force that is associated with fluids is the force exerted by fluids in motion known as drag force. Drag force is the force that opposes the motion of an object moving through a fluid.
Fluid forces can also be categorized as buoyancy force, the force that opposes an object's weight in a fluid. Buoyancy is determined by the density difference between the object and the fluid. An object that has a lower density than the fluid it is in will float, while an object that has a higher density than the fluid will sink.

1.4 Fluids and Free-Body Diagrams

Free body diagrams (FBDs) are a powerful tool for analyzing the forces acting on an object in a fluid system. At an AP Physics 2 level, students should understand how to use FBDs to analyze the forces acting on an object in a fluid system and how these forces relate to the behavior of the fluid.
A free body diagram is a simplified representation of an object and the forces acting on it. It is used to identify the forces acting on an object and to determine the net force, or the overall force acting on the object. To create a free body diagram, students should first identify the object and draw a simple diagram of it. Then, they should identify all the forces acting on the object and draw arrows pointing in the direction of the force, with the length of the arrow indicating the magnitude of the force.
When it comes to fluids, there are several important forces that should be considered in a free body diagram. These include the fluid weight force, the buoyancy force, and the drag force.
The fluid weight force is the force exerted on an object by the weight of the fluid acting on it. This force acts in the downward direction and is equal to the weight of the fluid that is displaced by the object.
The buoyancy force is the force exerted by a fluid on an object that opposes the object's weight. This force acts in the upward direction and is equal to the weight of the fluid that is displaced by the object.
The drag force is the force exerted by a fluid on an object that opposes the motion of the object. This force can act in any direction, depending on the direction of the fluid flow and the shape of the object.
To determine the net force acting on an object in a fluid system, students should add up all the forces acting on the object and then find the vector sum of the forces. This net force will determine the acceleration of the object, and can be used to solve for the motion of the object in the fluid system.

1.5 Buoyancy

Buoyancy is the upward force exerted on an object submerged in a fluid, which opposes the weight of the object.
The buoyancy force is caused by the pressure differences in the fluid that the object is submerged in. When an object is submerged in a fluid, the fluid exerts an upward force on the object, which is equal to the weight of the fluid that is displaced by the object. This upward force is called the buoyancy force.
The buoyancy force is determined by Archimedes' Principle, which states that the buoyancy force on an object submerged in a fluid is equal to the weight of the fluid that is displaced by the object. The buoyancy force can be calculated using the following formula:
F_b = \rho_f V g
Where F_b is the buoyancy force, \rho_f is the density of the fluid, V is the volume of the object submerged in the fluid, and g is the acceleration due to gravity.
The buoyancy force is affected by several factors, including the density of the fluid, the volume of the object submerged in the fluid, and the acceleration due to gravity. The density of the fluid is one of the most important factors that affect the buoyancy force, as the buoyancy force is directly proportional to the density of the fluid. The volume of the object submerged in the fluid also affects the buoyancy force, as the buoyancy force is directly proportional to the volume of the object submerged in the fluid. The acceleration due to gravity also affects the buoyancy force, as the buoyancy force is directly proportional to the acceleration due to gravity.

1.6 Conservation of Energy in Fluid Flow

Conservation of energy in fluid flow refers to the idea that the total energy of a fluid system remains constant, even as the fluid flows through different regions of the system. At an AP Physics 2 level, students should understand the concept of conservation of energy in fluid flow, as well as the key principles and formulas related to this concept.
One of the key principles related to conservation of energy in fluid flow is the principle of energy conservation. This principle states that the total energy of a closed system remains constant, even as the fluid flows through different regions of the system. This means that the energy that is added to the system in one region is equal to the energy that is lost in another region.
Another key principle related to conservation of energy in fluid flow is the Bernoulli's equation. This equation states that the sum of the kinetic energy, potential energy, and pressure energy of a fluid is constant along a streamline. This equation can be used to calculate the pressure and velocity of a fluid at different points in a system.
The key formula related to conservation of energy in fluid flow is the energy equation, which can be written as:
ΔE = Q + W
Where ΔE is the change in energy of the fluid, Q is the heat added to the fluid, and W is the work done on the fluid. This equation can be used to calculate the change in energy of a fluid as it flows through different regions of a system.

1.7 Conservation of Mass Flow Rate in Fluids

Conservation of mass flow rate in fluids refers to the idea that the mass of a fluid flowing through a system remains constant, even as the fluid flows through different regions of the system. At an AP Physics 2 level, students should understand the concept of conservation of mass flow rate in fluids, as well as the key principles and formulas related to this concept.
One of the key principles related to conservation of mass flow rate in fluids is the principle of continuity. This principle states that the mass flow rate of a fluid flowing through a system remains constant, as long as there is no net accumulation of mass within the system. This means that the mass flow rate at any point in the system is equal to the mass flow rate at any other point in the system.
The key formula related to conservation of mass flow rate in fluids is the continuity equation, which can be written as:
Q = A_1 V_1 = A_2 V_2
Where Q is the mass flow rate, A is the cross-sectional area, and V is the velocity of the fluid. This equation states that the product of the cross-sectional area and the velocity of the fluid is constant at any point in the system, as long as there is no net accumulation of mass within the system.
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