Pressure at a Point: Effects in All Directions?

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Discussion Overview

The discussion revolves around the nature of pressure in fluids, particularly how it acts in all directions at a point and its relationship to buoyancy. Participants explore the mechanisms behind pressure distribution in fluids, the concept of buoyant force, and the implications of gravity on pressure behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why pressure at a point affects all directions, contrasting it with gravity, which only acts downwards.
  • There is a suggestion that pressure may act upwards as a reaction to the weight of the fluid above.
  • One participant explains that pressure is exerted by the random motion of fluids and that buoyancy results from the difference in external pressure on an object.
  • Another participant illustrates the concept of pressure equilibrium in fluids by comparing it to squeezing an orange pip, noting that pressure from the sides balances the pressure from above.
  • Concerns are raised about understanding buoyant force, particularly how it arises from pressure differences between the top and bottom surfaces of an object in a fluid.
  • Some participants express confusion about the relationship between buoyancy and the concept of resistance in fluids, questioning the terminology used by educators.
  • There is a discussion about how pressure acts on the surfaces of an object, with some asserting that water pressure pushes down on the top surface and up on the bottom surface, while others challenge this view.
  • One participant emphasizes that pressure is a scalar quantity and does not have a direction, while forces derived from pressure can have directional components.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of buoyancy and pressure. Multiple competing views remain regarding the definitions and implications of pressure acting in different directions and the relationship between buoyancy and fluid resistance.

Contextual Notes

Participants express uncertainty about the definitions and implications of buoyancy and pressure, particularly in relation to the terminology used by educators. There are unresolved questions about the mechanics of pressure acting in various directions and how this relates to buoyancy.

Misr
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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
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:
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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.
 
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.
 
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.
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)
 
Misr said:
So gravity causes pressure in all directions?

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.
 
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...
 
The "buoyant 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).
 
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

*how is Pascal's principle related to Archimedes principle?
 
Last edited by a moderator:
Misr said:
Another question
water pushes also on the bottom surface of the object in the downwards direction as well as upward direction :right?
No. The water pressure pushes down on the top surface and up on the bottom surface.
 
  • #10
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.

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.
 
  • #11
Misr said:
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?

*how is Pascal's principle related to Archimedes principle?

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.
 
  • #12
Misr said:
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
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.

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.
 
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  • #13
No. The water pressure pushes down on the top surface and up on the bottom surface.
That's a very good point but why not ?Pressure affects in all directions at a certain point
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)
whatever the word we use I guess this is not a very scientific explanation
the water expels the the object only because of pressure difference not as a reaction to the object's weight.
Is that true?

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.
Yeah but I mean that the water above the bottom surface is pushing down on the bottom surface...not the water below this surface..
 
  • #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.
 
  • #15
just ignore that
I need answers to questions in post #13
 
  • #16
Misr said:
That's a very good point but why not ?Pressure affects in all directions at a certain point
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.
whatever the word we use I guess this is not a very scientific explanation
the water expels the the object only because of pressure difference not as a reaction to the object's weight.
Is that true?
Yes, buoyancy has nothing to do with the object's weight.
Yeah but I mean that the water above the bottom surface is pushing down on the bottom surface...not the water below this surface..
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.
 
  • #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?
 
  • #18
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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