IIT JEE , AIEEE Forum - Physics , Mechanics

anant_c

Suppose we have a fluid accelerating towards the positive direction of x axis...why is the pressure throught the fluid same.. in the sense that F(y)/A=F(x)/A

In short how do we prove that in general for a fluid(also an accelerating fluid) pressure is uniform from all directions?

anant_c

??????

anant_c

???????????

learner

This is just for a liquid at rest :

See the diagram please..

Aa/Ac = sin@

=>Aa = Acsin@

Ab/ac =cos@

=>Ab =Accos@

Resolve Fc

As fluid is at rest,

Fc sin @ =Fa

Fc cos @ =Fb

Fa/Aa=Fb/Ab

Thus, pressure at every point is the same..

Dunno about accelerating fluids...if pressure is the same, why will they flow??

Someone clarify please!

learner

Sorry,I forgot to mention...that thing is a body inside a fluid...

If pressure along all its sides is equla,it means pressure in all parts of the fluid is the same

anant_c

yea...i know this...this is in the NCERT buk....but for accelerating fluids...we use the condition i mentioned to calculate the angle made by the surface

hardcore_genius

Hey buddy! it's straight forward through Pascal's law. Now u wan 2 prove it, then in simple words i can tel u.
As th liquid is accelerating 2wards x axis, so the particles will strike the opposite wall & so pressure 'll increase.so, volume 'll decrease & temperature 'll increase ( 2 some extent). This 'll increase the K.E. of ptcles & so avg kE is increased for almost all partices & hence the pressure wil b equally distributed.
hope u understand............

elessar_iitkgp

Firstly, (no sarcasm intended) try phrasing your questions clearly.
If you are asking about a body placed in a fluid that is accelerating horizontally, then, pressure on the points of a body in the same straight line isn't the same, nor is the pressures at points separated vertically. For a fluid accelerating horizontally, isobaric surfaces are inclined planes

anant_c

wat i intended to ask is..that even in an accelerating fluid...is pressure on a point the same in all directions....if it is....how is it so...

thnx

learner

Thanks Elessar..that cleared my doubt...so the pressure wont be the same..

Incientally, @ hardcore genius, if pressure increases, how will volume decrease? You cant apply gas laws everywhere!!!

Besides, there is no mention of the fluid striking any surface,right?

anant_c

so is pressure along every direction same?

learner

No,it isnt.

If it was,the fluid would be stationary or in uniform motion.

anant_c

then how is it used in calculation of the angle the surface makes with the horizontal when the fluid accelerates in the +ve dirn of x axis

MUDIT

PLEASE CORRECT ME IF I AM WRONG
WHEN A FLUID ACCELERATES IN THE +VE X AXIS THERE IS PSEUDO FORCE ACTING IN THE NEGATIVE X DIRECTION WHICH INCREASES THE PRESSURE OF THE MOLECULES OF THE LIQUID ON THE RIGHT HAND SIDE OF THE BEAKER
SO A SITUATION COMES THAT IN A HORIZONTAL LINE THE MOLECULES TOWARDS THE RIGHT ARE AT A HIGHER PRESSURE WHICH IS NOT POSSIBLE AS THE BASIC LAWS SAY THAT EVERY MOLECULE ON A HORZONTAL LINE IN A LIQUID HAVE THE SAME PRESSURE.
HENCE TO MAKE PRESSURE ON THE RIGHT SIDE OF THE BEAKER EQUAL TO THE PRESSURE ON THE LEFT SIDE THE LIQUID ON THE LEFT SIDE RISES UP (PRESSURE INCREASES WITH DEPTH).

anant_c

I alter my question a bit....is the pressure difference same in the x and y directions?

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