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Pressure in Fluids

Pressure is force per unit area and increases with depth in an incompressible fluid.

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Pascal's Principle

Pascal?s Principle

Credit: Frank Kovalchek
Source: http://www.flickr.com/photos/72213316@N00/6624446347/
License: CC BY-NC 3.0

Hydraulic jacks, automobile brakes and even the lift generated on airplane wings can be explained using Pascal's principle. Pascal's principle is based on the idea that fluids at rest are incompressible, allowing very large forces to be transmitted with the application of a smaller force.

News You Can Use

  • Pascal's law was discovered during an experiment that was allegedly performed in the 1600?s. A 10 m long pole was vertically inserted into a barrel that was filed with water. As the water was poured into the vertical tube, Pascal discovered that the pressure from the water caused the barrel to burst.
  • Credit: Unknown
    Source: http://simple.wikipedia.org/wiki/File:Principe_de_Pascal.jpg
    License: CC BY-NC 3.0

    As water is poured into the top of the vertical tube, the barrel begins to burst [Figure2]

  • The modern equivalent of this can be seen in any hydraulic automobile jacks present in some household garages. Two pistons are connected together by a fluid; one piston is much smaller than the other. A small force is applied to the small piston that produces a change in pressure which is transmitted to the fluid. This pressure is given as \begin{align*}\frac{F_1}{A_1}\end{align*}, where \begin{align*}F_1\end{align*} is the applied force and \begin{align*}A_1\end{align*} is the area of the small piston. Since the pressure at the small end must equal the pressure at the large end, it is easily seen that the force on the larger piston is:

\begin{align*}F_2 = \left(\frac{A_2}{A_1}\right)F_1\end{align*}

  • Discover more about hydraulic lifts: 


Show What You've Learned

Using the information provided above, answer the following questions.

  1. If you were to place a car on piston A in the video instead of piston B, would you have to apply a larger or smaller force to piston B to raise the car?
  2. Why does Pascal's law not work if you replace the fluid in a hydraulic lift with CO2 gas?
  3. In the experiment that was performed by Pascal, why did the barrel burst?

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Image Attributions

  1. [1]^ Credit: Frank Kovalchek; Source: http://www.flickr.com/photos/72213316@N00/6624446347/; License: CC BY-NC 3.0
  2. [2]^ Credit: Unknown; Source: http://simple.wikipedia.org/wiki/File:Principe_de_Pascal.jpg; License: CC BY-NC 3.0

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