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Atmospheric pressure - questions

  1. Jul 16, 2014 #1
    I have two simple questions.

    1) I don't understand atmospheric force. I read that it occurs by heaviness of air column and it act perpendicular to any surface.

    I can't imagine it. Where is the air column in experiment with glass and papper? Why the atmospheric force act perpendicular to any surface?

    2) You have some plastic bottle and you make some holes to it. Than you remove the plug of the bottle. Now you see the springs of watter going on parabole down.

    Why? OK, i read if the plug is on the bottle, the atmospheric pressure is bigger than hydrostatic pressure, so watter don't go out from holles. However, if I remove the plug, hydrostatic pressure is bigger. Can you tell me why is bigger now?
  2. jcsd
  3. Jul 20, 2014 #2
    Atmospheric pressures are the pressures that the molecules of atmospheric gases exert upon any exposed surface due to molecular impacts with that surface. This pressure (in Pascals per square meter) is the simple product of the molecular flux (in mean number of molecular impacts per square meter per second) and the mean impulse per impact (in Newtons). Pressure equals the molecular flux times the mean molecular impulse.

    I don't really understand your other references or questions. I suspect that English is not your first language.
  4. Jul 20, 2014 #3


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    For your 2nd question, the hydrostatic pressure is the same, however when u remove the plug you also have the air from the top of the bottle pressing the water down , so the water at the holes is pushed to go outside by air pressure+hydrostatic pressure and is pushed to stay inside only by the surrounding to bottle air pressure. The difference in those pressures is the hydrostatic pressure which makes the water go out.

    For your 1st question, i think what you missing is that pressure is a scalar quantity (pressure doesnt have a direction like a force has, pressure has only magnitude) and the force is equal to the gradient of pressure. The gradient of pressure is a vector quantity.
    Last edited: Jul 20, 2014
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