The very first unit of AP Physics 2 begins with a look at fluids. In the first year physics course, we mostly looked into the kinematics, dynamics, and interactions of solids. We often ignored the internal aspects of the system and considered the object as ideal and non-deformable. We begin this course by looking at fluids, which consist of gases and liquids, and their interactions🌪️ 

As you already know, matter mostly comes in three forms: solids, liquids, and gases. In Physics 1, we mostly dealt with solids. In this unit, we shift our focus to liquids. 🚿

What are Fluids?

You can remember a fluid as anything that flows. Therefore, both liquids and gases are considered fluids.

One thing to remember about fluids is that they do not have a definite shape. Unlike solids, liquids and gases can change their shape based on the container or the area we bind them in.

Example of fluids: water (most common), oil, oxygen, helium, air etc. 

Fluids have an internal structure that we cannot ignore. Therefore, the type of liquid we have will be very important for our calculations and processes!

Here are some key points to remember about fluid systems:

• A fluid system is a system that involves the flow of a fluid, such as a gas or liquid.
• The properties of a fluid, such as its density, viscosity, and surface tension, can affect its behavior in a fluid system.
• Fluid systems can be classified as either open or closed, depending on whether the fluid can flow in and out of the system.
• The behavior of a fluid in a system can be described using principles of fluid mechanics, such as the relationship between pressure and volume, or the Bernoulli equation.
• Fluid systems can be studied at different scales, from microfluids (small-scale systems involving the flow of fluids through narrow channels or pores) to macrofluids (large-scale systems involving the flow of fluids through pipes or tanks).
• Fluid systems can be found in a wide variety of applications, including transportation (e.g. air and water flow in pipes or channels), energy production (e.g. flow of fluids in power plants or oil wells), and biology (e.g. flow of blood through the circulatory system).

Example Problem:

How do the shape and size of a container affect the behavior of a fluid inside it? For example, how does the volume of a container influence the pressure that a fluid exerts on the walls of the container?

Object vs System

Another important distinction that comes up in this section is the idea of object vs system. An object does not have an incredibly precise definition, but it can be generalized as a collection of matter. A system, then, can be said to be a collection of objects.

Since objects make up systems, the properties of the objects determine the behavior of the system. These properties refer to the internal structure of the system. The internal structure plays a much bigger role when discussing the physics of fluids then it does with solids.

In some cases, the internal structure of the system is not extremely important to the macroscopic behavior of a certain model. In such a case, the system itself can be thought of as an object.

Here are some key points about the difference between an object and a system:

• An object is a single physical entity that is separate from its surroundings. It can be an individual component or a group of components that are combined in a specific way.
• A system is a group of objects or components that interact with each other and with their environment in a specific way. The behavior of the system is determined by the interactions between its components and the rules that govern those interactions.
• In the context of fluids, an object could be a single component of a fluid system, such as a pipe or a valve. A system, on the other hand, would be the combination of multiple objects or components that form a fluid system, such as a pipeline or a pump.
• The behavior of an object is determined by its own characteristics and properties, such as its shape, size, and material. The behavior of a system, on the other hand, is determined by the interactions between its components and the rules that govern those interactions.
• For example, the flow of a fluid through a pipe is an object-level phenomenon, while the behavior of a pump that transports the fluid from one location to another is a system-level phenomenon. Understanding both the object-level and system-level behaviors is important for designing and analyzing fluid systems.

Let’s try to look at an example. 

Let’s say we have a balloon filled with some gas. The objects in this example would be the gas in the balloon and the balloon itself. The gas-balloon entity would be our system. If we were trying to just model the general behavior of the system, for example finding the temperature of the system, we could just say that our system - the balloon-gas combination - is our object.

This semantics discourse is not incredibly important but will enhance your overall ability to understand physics and its fundamentals. 

Some common liquids you will be working with include water, salt water, and oil. Be careful salt water and water have different densities and the AP exam likes to ask about both!