Fluid pressure and buoyancy might seem like complicated physics concepts,... Show more
AP Physics 1173 views·Updated May 27, 2026·3 pagesAP Physics 2: Fluid Pressure, Buoyancy, and Principles Explained
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Fluid Pressure Basics
Ever wonder why your ears hurt when you dive deep in a pool? That's fluid pressure in action! Pressure is simply force spread over an area, measured in Pascals (Pa).
There are two main types of pressure you need to know. Absolute pressure measures pressure compared to a complete vacuum (like outer space). Gauge pressure measures pressure compared to the air pressure around us.
Here's the key formula: Pabsolute = Po + ρgh, where Po is the initial pressure above the fluid. For gauge pressure, it's just Pgauge = ρgh. The deeper you go in any fluid, the more pressure you feel!
Quick Tip: Atmospheric pressure is about 1.01 × 10⁵ Pa - that's the weight of all the air above us pressing down!
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Buoyancy and Archimedes' Principle
Why do some objects float while others sink? It's all about the buoyant force - the upward push that fluids give to objects. This happens because pressure increases with depth, creating more force on the bottom of an object than the top.
Archimedes' Principle gives us the secret: the buoyant force equals the weight of the fluid that gets displaced (pushed out of the way). The formula is FB = ρgVdisplaced, where you use the fluid's density, not the object's density.
Here's how it works in real life: if the buoyant force is stronger than the object's weight, it floats (positive buoyancy). If the object weighs more, it sinks (negative buoyancy). When they're equal, the object hovers (neutral buoyancy) - like a submarine underwater!
Remember: A floating object displaces exactly enough fluid to equal its own weight - that's why ice cubes don't overflow your drink!
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Pascal's Principle and Hydraulics
Here's something cool: when you squeeze a water bottle, the pressure increases everywhere inside, not just where you're pushing. That's Pascal's Principle - any pressure change in an enclosed fluid spreads equally throughout the entire fluid.
This principle powers amazing machines like hydraulic car lifts and brakes. The math is simple: F₁/A₁ = F₂/A₂. A small force applied to a small area can create a huge force over a large area.
Think about it this way - hydraulic systems are like mechanical advantage machines for fluids. You can push down hard on a small piston to lift something really heavy with a larger piston. It's the same reason why car brakes work so effectively when you press the brake pedal!
Real World Connection: Every time you use a hydraulic car jack or press your car's brake pedal, you're using Pascal's Principle to multiply force!
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AP Physics 1173 views·Updated May 27, 2026·3 pagesAP Physics 2: Fluid Pressure, Buoyancy, and Principles Explained
Fluid pressure and buoyancy might seem like complicated physics concepts, but they're actually everywhere around you - from swimming pools to hydraulic car lifts. Understanding how pressure works in fluids and why objects float or sink will help you make... Show more
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Fluid Pressure Basics
Ever wonder why your ears hurt when you dive deep in a pool? That's fluid pressure in action! Pressure is simply force spread over an area, measured in Pascals (Pa).
There are two main types of pressure you need to know. Absolute pressure measures pressure compared to a complete vacuum (like outer space). Gauge pressure measures pressure compared to the air pressure around us.
Here's the key formula: Pabsolute = Po + ρgh, where Po is the initial pressure above the fluid. For gauge pressure, it's just Pgauge = ρgh. The deeper you go in any fluid, the more pressure you feel!
Quick Tip: Atmospheric pressure is about 1.01 × 10⁵ Pa - that's the weight of all the air above us pressing down!
2
of 3Sign up to see the content. It's free!
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Buoyancy and Archimedes' Principle
Why do some objects float while others sink? It's all about the buoyant force - the upward push that fluids give to objects. This happens because pressure increases with depth, creating more force on the bottom of an object than the top.
Archimedes' Principle gives us the secret: the buoyant force equals the weight of the fluid that gets displaced (pushed out of the way). The formula is FB = ρgVdisplaced, where you use the fluid's density, not the object's density.
Here's how it works in real life: if the buoyant force is stronger than the object's weight, it floats (positive buoyancy). If the object weighs more, it sinks (negative buoyancy). When they're equal, the object hovers (neutral buoyancy) - like a submarine underwater!
Remember: A floating object displaces exactly enough fluid to equal its own weight - that's why ice cubes don't overflow your drink!
3
of 3Sign up to see the content. It's free!
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Pascal's Principle and Hydraulics
Here's something cool: when you squeeze a water bottle, the pressure increases everywhere inside, not just where you're pushing. That's Pascal's Principle - any pressure change in an enclosed fluid spreads equally throughout the entire fluid.
This principle powers amazing machines like hydraulic car lifts and brakes. The math is simple: F₁/A₁ = F₂/A₂. A small force applied to a small area can create a huge force over a large area.
Think about it this way - hydraulic systems are like mechanical advantage machines for fluids. You can push down hard on a small piston to lift something really heavy with a larger piston. It's the same reason why car brakes work so effectively when you press the brake pedal!
Real World Connection: Every time you use a hydraulic car jack or press your car's brake pedal, you're using Pascal's Principle to multiply force!
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