Fluid physics is all about how liquids and gases behave... Show more
AP Physics 156 views·Updated May 26, 2026·2 pagesAP Physics 2: Fluids - Fundamentals and Applications
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Fluid Systems
When fluid molecules are in a container, they constantly collide with the walls. While random collisions create forces in all directions, the parallel forces cancel out, leaving only the perpendicular forces. This means fluid pressure always acts perpendicular to any surface it contacts.
A stationary fluid is in equilibrium, with forces balanced at all points. Imagine two points in a container of liquid—point A near the top and point B deeper down. Point B experiences greater pressure because it must support more fluid weight above it than point A does. This explains why you feel increasing pressure as you swim deeper in water.
Did you know? The pressure you feel at just 10 feet underwater is noticeably greater than at the surface—this is why your ears might "pop" when diving into a pool!
Both liquids and gases are considered fluids, though they behave differently. Liquids are typically "incompressible" (they maintain their volume when pressured) while gases are "compressible" (their volume changes under pressure). This distinction becomes important when analyzing how fluids interact with other objects.
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Density
Density is one of the most important fluid properties, calculated as mass divided by volume , measured in kg/m³. This simple ratio tells us how much "stuff" is packed into a given space, helping us predict how objects will behave in fluids.
Specific Gravity (SG) compares an object's density to water's density: SG = ρ_object/ρ_water. This gives us a quick way to predict floating behavior—if SG > 1, the object sinks; if SG < 1, it floats. You can apply this concept with any fluid, not just water.
Remember this: The weight of a fluid can be calculated as ρVg (density × volume × gravity), while an object's weight is simply mg (mass × gravity).
When you know an object's specific gravity relative to a fluid like oil, you can immediately predict whether it will sink or float in that particular fluid. This concept helps explain everyday phenomena like why ice cubes float in water but sink in alcohol.
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AP Physics 156 views·Updated May 26, 2026·2 pagesAP Physics 2: Fluids - Fundamentals and Applications
Fluid physics is all about how liquids and gases behave when at rest or in motion. Understanding fluid concepts helps explain everything from why you feel more pressure at the bottom of a swimming pool to why some objects float... Show more
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Fluid Systems
When fluid molecules are in a container, they constantly collide with the walls. While random collisions create forces in all directions, the parallel forces cancel out, leaving only the perpendicular forces. This means fluid pressure always acts perpendicular to any surface it contacts.
A stationary fluid is in equilibrium, with forces balanced at all points. Imagine two points in a container of liquid—point A near the top and point B deeper down. Point B experiences greater pressure because it must support more fluid weight above it than point A does. This explains why you feel increasing pressure as you swim deeper in water.
Did you know? The pressure you feel at just 10 feet underwater is noticeably greater than at the surface—this is why your ears might "pop" when diving into a pool!
Both liquids and gases are considered fluids, though they behave differently. Liquids are typically "incompressible" (they maintain their volume when pressured) while gases are "compressible" (their volume changes under pressure). This distinction becomes important when analyzing how fluids interact with other objects.
2
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Density
Density is one of the most important fluid properties, calculated as mass divided by volume , measured in kg/m³. This simple ratio tells us how much "stuff" is packed into a given space, helping us predict how objects will behave in fluids.
Specific Gravity (SG) compares an object's density to water's density: SG = ρ_object/ρ_water. This gives us a quick way to predict floating behavior—if SG > 1, the object sinks; if SG < 1, it floats. You can apply this concept with any fluid, not just water.
Remember this: The weight of a fluid can be calculated as ρVg (density × volume × gravity), while an object's weight is simply mg (mass × gravity).
When you know an object's specific gravity relative to a fluid like oil, you can immediately predict whether it will sink or float in that particular fluid. This concept helps explain everyday phenomena like why ice cubes float in water but sink in alcohol.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content in AP Physics 1
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user