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Criteria for a fluid being under 'pressure'

  1. Sep 29, 2009 #1
    A fluid is said to be in pressure if it has the ability to apply a force on an object and if the object is placed in the fluid or at the boundary of the fluid and it's surroundings.

    Is this force application required to be in all directions if a fluid is considered to be under pressure?...I mean suppose a fluid contains some amount of K.E by virtue of motion in the y axis then this fluid has the ability to apply force on an object which is placed in it's path...but the force will only be applied in one direction on the object...so will this moving fluid be considered under pressure?...or is it that the fluid will be said to be in pressure only if it has the ability to apply force on all sides of the object which is contained in it?
  2. jcsd
  3. Sep 29, 2009 #2
    By this definition, the fluid will be under pressure when it hits the object it is facing. Think about it like this, pressure in a fluid is a result of all the molecules of that fluid bouncing around and hitting each other. So when you stand against the wind, the force you feel is actually the constituent particles of the wind (air is also a fluid) hitting against your body.

    Fluid pressure has nothing to do with another body being present or not. Even at rest, a fluid is exerting pressure on whatever is below it. Although you may not notice it, everytime you step outside in the open, you are experiencing the pressure of a tall column of atmosphere which lines up on top of your head. you just dont notice it because its usual.

    I think you are confusing two things here. I think your concept of "all directions" comes from what Pascal's law states? But no, a fluid does not need to apply a force to something in all directions to exert a pressure on it. Pressure is simple force per unit area. As long as there's a non zero force acting on a finite area, there is pressure.
  4. Sep 30, 2009 #3
    Ok, so it's not under pressure in this condition...hummm.

    In the scenario when the fluid has K.E, the fluid molecules wont hit each other.
    To satisfy the criteria of pressure being in a fluid the force that falls on the submerged object should be from all directions, and should be equal in all the directions if the fluid is not under influence of a field (very popularly, gravity).

    If the fluid is under influence of a field (like the sea under influence of gravity) the force from all directions will not be equal...but we still classify our atmosphere as being under pressure.

    Thus the criteria for a fluid being under pressure should be that it's molecules should apply force on each other.

    This will not be classified as pressurized fluid cause here the molecules of the fluid are not applying force on each other.

    Another definition...I think can be that a pressurized fluid will apply force on an object contained in it even if the net velocity of the fluid and the object is the same...which is not in this wind's case...so we wont call the wind as fluid under pressure.

    Yes, I know, I was and am just taking it as a test.

    On the whole body in general...not just the head :D

    Yes, as stated before, this is the reason -

    I hope I'm right...and I got a feeling I'm not.
  5. Sep 30, 2009 #4
    That kinda sums it up. Whether there is equal pressure on all sides of an object or not, fluids exert pressure when the sum total of its molecules exert a non zero force on a surface which may be exterior to it, or may be an imaginary surface that cuts through the fluid bulk. But be warned, the only condition in which the molecules of a fluid will completely stop exerting force on each other (i.e. stop bouncing around) is when the fluid is at rest thermodynamically. (something like zero Kelvin maybe)

    Why do you think so? Even when the overall fluid has a K.E., the velocity of its individual molecules are in no way equal to the external velocity of the fluid as a whole. You see, the molecules inside are randomly bouncing around and hitting each other. When a fluid has a macroscopic velocity, again it means that the sum total of the molecular velocities add up in a certain direction with some certain value. I could go into the gory details giving examples but that would make me digress.

    Not necessarily. Think of a sunken ship. The base of the ship is touching the ground and say, in an ideal condition, the force on it from the bottom is much much smaller as compared to the force on the ship coming from the top. Thats one reason its still at the bottom. If all the forces on the submerged body would be equal in all directions, it would just be stationary inside the fluid. kinda like a spaceship floating in our atmosphere.


    Ok, so a pressurized fluid is simply a quanta of fluid which is *under* pressure. So yeah the wind case is not a pressurized fluid case.

    There is a difference between a pressurized fluid and a fluid exerting pressure. I was talking about a fluid exerting pressure all this while.

    The way I learnt to get around this whole anomaly of being under pressure and applying pressure was this: Forget the common understanding of pressure being something that eventually produces a force. Think of pressure like this. Imagine a room, now imagine you can bring in one molecule of air at a time into the room. As soon as you get the first molecule in, the air inside will start exerting pressure on the room's walls. So pressure is just dependent on the no. of molecules you have inside a volume. The higher no. of molecules, the more tightly packed they are and the more pressure they can potentially exert on either themselves, or an external wall
  6. Oct 3, 2009 #5
    In this case the in internal collisions among the molecules is not by virtue of the K.E but other reasons...like the atmospheric pressure, temperature etc...

    But definitely not by virtue of K.E.


    Actually (as we all know) in any body which is sunk in the sea, the pressure can never be equal in all directions unless made a hypothetical case (a plane).

    The pressure that you're talking about is here -

    Is cause of the intermolecular repulsion...when we talk about sea...or even the atmospheric pressure its primarily cause of a field (gravity)...which actually is acting against the intermolecular repulsion (this highly depends on the fluid under question).
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