Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Pressure at points on the same horizontal plane

  1. Oct 9, 2011 #1
    As you know,pressure is the same at points on the same horizontal plane
    Okay,suppose two points A,B in a fluid are on the same horizontal plane
    [PLAIN]http://img197.imageshack.us/img197/1977/unledfhe.jpg [Broken]
    the pressure on both points is the same
    but at A pressure=h2*ρg
    pressure at B=h1ρg
    and h1 is not equal to h2 as you see
    could you explain this
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 9, 2011 #2
    Pressure is rho g h right? So the water in the middle has pressure applied on it from atmosphere as well as the water above it, while the water to the left only has pressure applied to it by the water above it. Pressure does not spread laterally.

    If you push on a piece of paper, the paper around the paper you pushed on feels some of the pressure exerted too. This is not the case with water. Each molecule is very small and has a lot of freedom to move. Imagine that each water molecule in the middle has some force exerted on it because of pressure. You think that this force might cause those molecules to push on molecules to the left and right, but this is not the case.

    A simpler explanation is that the pressure in this case is caused by gravity, and gravity doesn't exert a force sideways.
  4. Oct 9, 2011 #3
    Actually, fluids - liquids and gases - transfer whatever pressure applied to them in all directions...downwards, sideways, upwards...

    The atmospheric pressure out on the yard is 1 atm...when you snick back in the porch with a ceiling only 10 feet high...is the pressure a lot less? No.

    If the pressure in A was not the same as the pressure in B...you could dive to the bottom of the ocean, carve a hole on a wall and a bit of ceiling to snick under and all of a sudden have minimal pressure? I don't think so.
  5. Oct 11, 2011 #4
    so what is the pressure in the case above?
  6. Oct 11, 2011 #5
    so what is the pressure in the case above?
  7. Oct 11, 2011 #6
    so the extra pressure at point B is transfered to point A untill they are equal in pressure??

    but the pressure is much less at high mountains
  8. Oct 11, 2011 #7
    The pressures at A and B are the same.

    Whether pressure is less at high mountains has not much to do with the problem at hand...you are talking about pressure between two points at the same depth inside a fluid.

    But, yes, pressure at high mountains is less because such point is less deep measuring from the top of the atmosphere, down.
  9. Oct 11, 2011 #8
    Ok what makes pressure the same at A and B?
  10. Oct 11, 2011 #9


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Gravity doesn't need to "act sideways" to produce a pressure to the side any more than a crank lever needs to be pushed in the same direction as the direction you want a force to be applied. Fluid will flow from a high pressure region to a low pressure region - in any direction. If you did the experiment out in zero g, the pressures would all be equal everywhere in the container (cabin pressure or more if you squeeze on a sealed container). Back on Earth, there is a hydrostatic effect that causes the pressure at any level to increase with depth. This pressure will be acting in all directions so fluid will move (microscopically) from B towards A until equilibrium is reached. Then the pressure will be the same over all that particular level. If you were, suddenly, to add another metre of liquid to the vertical tube, there would be a slight delay before the wave of increased pressure travelled from B to A and equilibrium was again reached. (Speed of sound in the liquid)
  11. Oct 11, 2011 #10

    Doc Al

    User Avatar

    Staff: Mentor

    Not true.
    True (neglecting atmospheric pressure).
    h1 is the depth below an open surface, but h2 is not. The upper surface of the container above A exerts a downward pressure that must be added to h2*ρg to get the actual pressure at A. The net effect is that the pressure is equal at A and B.
  12. Oct 12, 2011 #11
    Yeah,that's great and easy to imagine
    thanks very much
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook