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Pressure in Liquids and Pascal's Principle?

  1. Oct 27, 2012 #1
    Okay, so Pascal's Principle (as copied from Wikipedia):

    A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.

    From what I've gathered, this means that pressure in liquids act in all directions, so if you press a closed container filled with water, then the pressure will spread and act in all directions. However, how does this work? If you press down on it with a downwards force, then won't the pressure act downwards instead of in all directions?

    In still water, for example, in the ocean, the weight (gravity) of the water is also a "downwards" which means that the pressure is exerted downwards and not in all directions?

    I've tried visualizing this using particles, but it doesn't really make sense- if you press downwards then the particles are moving downwards which collides with other particles and makes them move downwards, so in the end there's only pressure on the bottom of the container and not on the sides of the container?

    I'm really confused now. :(

    Thanks in advance for your help.
  2. jcsd
  3. Oct 27, 2012 #2
    The force per unit area on the vertical walls of the container, or on vertical surfaces at the bottom of the ocean is the same as the force per unit area on a horizontal surface, such as the bottom of the container, or the ocean floor. The pressure of the overlying weight is transmitted equally in all directions. Did you ever swim to the bottom of a pool at the deep end and feel the increased pressure on your ear drums?

    I get the feeling that you think some kind of force balance has to apply to the horizontal direction in order for the pressure to be high in that direction. But, if you do a free body diagram on a cube of water at depth, there is nothing that says that the horizontal forces on the cube can't be high, while at the same time preserving equilibrium.
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