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Pressure at a point

  1. Oct 20, 2011 #1
    why does pressure at a point affects in all directions?
    gravity only affects downwards,so what makes pressure affect in all directions?

    may be the pressure affects upwards as a reaction to the fluids weight(that affects downwards)
    [PLAIN]http://img208.imageshack.us/img208/9738/unled123.jpg [Broken]
    so, what about this lateral pressure?where does this lateral pressure come from?

    I also don't understand
    how does buoyant force result from the difference in pressure between the surface of the cylinder in the fluid and its base?I can't imagine this at all.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 20, 2011 #2

    russ_watters

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    Pressure is exerted by the random motion of fluids. Random.

    Buoyancy results from the difference in external pressure between the bottom and top surfaces of an object. Although in your object's case, it appears to be an open-topped vessel and thus has no buoyancy.
     
  4. Oct 20, 2011 #3

    sophiecentaur

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    A force in one direction can easily produce a force in another direction. Think of what happens when you squeeze an orange pip between your fingers. Pushing down on a fluid will cause it to spread. it will spread until the pressure from the sides equals the pressure from above. An equilibrium situation will be reached in which pressures are all equal.
     
  5. Oct 20, 2011 #4
    So gravity causes pressure in all directions?
    then I was not right when i said that the pressure affects in the upward direction as a reaction to the fluids weight(that affects downwards)
     
  6. Oct 21, 2011 #5

    sophiecentaur

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    In a fluid, yes. Push on the handle of a pump (force from one direction) and the pressure inside (all directions) goes up. That's a pretty familiar experience.
     
  7. Oct 21, 2011 #6
    That's good.but how about buoyant force? we study that buoyant force results from the difference in pressure between the surface of the cylinder in the fluid and its base
    I can't imagine this at all.

    We also study that the buoyancy is the reaction of the fluid to the object pushing on it

    I can't relate all of this together...
     
  8. Oct 21, 2011 #7

    sophiecentaur

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    The "bouyant force" is just the net sum of the vertical components of the pressure on an object that is immersed in a fluid or even suspended between two fluids (as in floating in water with the top bit in air). There will be pressure downwards and there will be pressure upwards but, because of the volume of the object, the downwards pressure will be less than the upward pressure, even when the object is totally under water. Imagine breaking the object down into a number of vertical cylinders and then consider the difference in the pressure on each top face and each bottom face.
    That's where your upward force comes from and it happens to be equal to the Weight of the fluid that the object has displaced whatever orientation the object happens to be in (Cheers Mr. Archimedes).
     
  9. Oct 23, 2011 #8
    Yeah,that's a good explanation.but why do some teachers consider buoyancy as the resistance of the fluid to the object or the reaction of the fluid to the object pushing on it..?Is that true?

    or may be they just say so because the fluid pushes the object up by the effect of buoyancy

    Another question
    *water pushes also on the bottom surface of the object in the downwards direction as well as upward direction :right?so what makes buoyancy?
    [PLAIN]http://img696.imageshack.us/img696/3654/unled22rg.jpg [Broken]

    *how is Pascal's principle related to Archimedes principle?
     
    Last edited by a moderator: May 5, 2017
  10. Oct 23, 2011 #9

    Doc Al

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    No. The water pressure pushes down on the top surface and up on the bottom surface.
     
  11. Oct 23, 2011 #10
    Pressure equals height time density: P = hd....so at the bottom of an object the pressure is higher than at the top because there is greater height...

    If you don't recognize this, fill a uniform beaker half full of water and weigh it......then double the height of the fluid...it will weigh twice as much.....there is twice the weight (and volume) of water pushing down on the bottom of the tube...against the scale....more pressure.
     
  12. Oct 23, 2011 #11

    sophiecentaur

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    I would be very reluctant to use the word "resistance" rather than "reaction", for a start. To my mind, resistance implies energy loss (as in electrical resistance).

    Also, how can a fluid 'push down" on the lower surface of the object? The water just below the bottom of the object is pushing down on the water below it. - contributing to the pressure lower down.
     
  13. Oct 23, 2011 #12
    Gravity is a force [*] and a force is associated to a vector. A vector has a direction.

    Pressure is a scalar. Scalars have no direction. You can obtain the force F in a given direction using pressure P and the surface A perpendicular to that direction.

    F = P A

    [*] Not in general relativity.
     
    Last edited by a moderator: May 5, 2017
  14. Oct 24, 2011 #13
    That's a very good point but why not ?Pressure affects in all directions at a certain point


    whatever the word we use I guess this is not a very scientific explanation
    the water expels the the object only beacause of pressure difference not as a reaction to the object's weight.
    Is that true?

    Yeah but I mean that the water above the bottom surface is pushing down on the bottom surface...not the water below this surface..
     
  15. Oct 24, 2011 #14
    "Another question
    water pushes also on the bottom surface of the object in the downwards direction as well as upward direction right? so what makes buoyancy?"
    Nothing, the small container in your picture is full of water so isn't buoyant!
    Did you really need someone to tell you that?

    Obviously, if the container is full of air then there isn't a water pressure pushing down on the bottom surface.
     
  16. Oct 30, 2011 #15
    just ignore that
    I need answers to questions in post #13
     
  17. Oct 30, 2011 #16

    russ_watters

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    Sure, but it can't push up on something below it!! The pressure at a point pushes up on what is above it and below on what is below it. That's what it means to act in every direction.
    Yes, buoyancy has nothing to do with the object's weight.
    Huh? The water below the bottom surface can only push up on the object since the object is above it. Water above the bottom surface doesn't make sense. Are you saying it is full of water? Even if it is, it is irrelevant to its buoyancy.
     
  18. Nov 1, 2011 #17
    I think that the confusion appears because we think of the gravitational pull on the liquid above the object, as if it were a macroscopic object (with mass according to the density of the liquid). Then we start talking about pressure as the result of microscopic motion of molecules. Maybe the missing link would be to explain how the motion of the molecules, and therefore the pressure, depends on the depth?
     
  19. Nov 1, 2011 #18

    sophiecentaur

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    There is often a real problem in teetering on the brink between microscopic and macroscopic worlds. However, you could always consider the individual molecules gaining KE / momentum as they fall down and losing it as they rise - giving rise to greater pressure on the bottom face of an elemental layer than on the top. The difference in KE would correspond to the difference in GPE from top to bottom - which yields ρgΔh as the pressure differential, after a bit of jiggery pokery.
     
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