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Pressure in Fluids

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Pascal's Principle
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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.
  • The modern equivalent of this can be seen in any hydraulic automobile jacks seen in most household garages. There are two pistons that are connected together by a fluid where one is much smaller than the other. A small force is applied on the small piston that produces a change in pressure that is transmitted to the fluid. This pressure is given as \frac{F_1}{A_1}, where A_1 is the area of the small piston. Since the pressures at the small end must equal the pressure at the large end, it is easily seen that the force on the larger piston is

F_2 = \left(\frac{A_2}{A_1}\right)F_1.

  • Discover more about hydraulic lifts: 

http://www.youtube.com/watch?v=A3ormYVZMXE

Show What You 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 replaced the fluid in a hydraulic lift with CO2 gas?
  3. In the experiment that was performed by Pascal, why did the barrel burst?

Image Attributions

  1. [1]^ Credit: Frank Kovalchek; Source: http://www.flickr.com/photos/72213316@N00/6624446347/; License: CC BY-NC 3.0

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