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Fluid Dynamics

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More Lessons for High School Physics

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A series of free Online High School Physics Video Lessons.

In this lesson, we will learn

  • Atmospheric Pressure
  • Barometer
  • Continuity Equation
  • Siphoning - Torricellis Law

Atmospheric Pressure

Atmospheric pressure is the pressure caused by the mass of our gaseous atmosphere. It can be measured using mercury in the equation atmospheric pressure = density of mercury × acceleration due to gravity × height of column of mercury. Atmospheric pressure can be measured in atm, torr, mm Hg, psi, Pa, etc.

Understanding atmospheric pressure versus gauge pressure and total pressure.
Can-crushing, balloon-inflating, juice-box-drinking action in this study of air pressure: how it happens, what it is, and why


A barometer is a device used to measure pressure. Gauge pressure is the difference between a pressure and atmospheric pressure. Gauge pressure can be negative while total pressure cannot.

How to use barometers to measure pressure.
We'll learn about the amount of pressure that the air around us exerts, and we'll see how to measure pressure using a U tube barometer or a manometer with a vacuum. The units that we get are in mmH2O and mmHg. The amount of pressure changes depending on the altitude above or below sea level.


Continuity Equation

The continuity equation deals with changes in the area of cross-sections of passages which fluids flow through. Laminar flow is flow of fluids that doesn't depend on time, ideal fluid flow. The formula for continuity equation is density1 × area1 × volume1 = density2 × area2 × volume2.

An explanation of the Continuity Equation.

Bernoulli's Principle

Bernoulli's Principle replaces Pascal's Principle and liquid pressure for flowing fluid. It states that as you increase a fluid's speed, you decrease its the pressure that fluid exerts. Stagnant fluid exerts higher pressure than flowing fluid. Airplanes can fly because the way their wings are designed create pockets of stagnant air beneath the wings allows airplanes to fly.

An explanation of Bernoulli's Principle.
Bernoulli Effect - one of the principles involved in helping airplanes fly.

Siphoning - Torricellis Law

Siphons are tubes which draw fluid over the rim of a tank to a lower point. After an initial pressure change to initiate the flow, siphons operate continuously due to the pull of gravity. Torricelli's law says if a liquid flows from an opening in a container, its speed is the same as the speed that fluid would have if it dropped in free fall from the top of that container. Torricelli's Law derives directly from Bernoulli's principle.

This video takes a look at and explains how a siphon works.

Demonstration of Torricelli's law


Rotate to landscape screen format on a mobile phone or small tablet to use the Mathway widget, a free math problem solver that answers your questions with step-by-step explanations.

You can use the free Mathway widget below to practice Algebra or other math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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