plz give me the whole derivation of pascal's law...

Consider a very small right angled triangular element ABC of a liquid as shown in figure.

23547/pascal.png
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Let:
  • p_{x} = Intensity of horizontal pressure on the element of the liquid
  • p_{y} = Intensity of vertical pressure on the element of the liquid
  • p_{z} = Intensity of pressure on the diagonal of the triangular element of the liquid
  • theta = Angle of the triangular element of the liquid
Now total pressure on the vertical side AC of the liquid, Similarly,total pressure on the horizontal side BC of the liquid, and total pressure on the diagonal side AB of the liquid, Since the element of the liquid is at rest, therefore sum of the horizontal and vertical components of the liquid pressure must be equal to zero. Now using eqilibrium condition for horizontal pressure,
P_{z}sin theta  = P_{x}
Rightarrow p_{z}.AB.sin theta  = p_{x}.AC
From the geometry of the figure, we find that,
ABsin theta  = AC
therefore p_{z}.AC  = p_{x}.AC
Now using equilibrium condition for vertical pressure, i.e.,
p_{z} cos theta  = p_{y}-W
(where W = Weight of the liquid) As the triangular element is very small, the weight of the liquid W is neglected, so,
p_{z} cos theta  = p_{y}
therefore p_{z}.AB.cos theta  = p_{y}.BC
From the geometry of the figure, we find that
ABcos theta  = BC
therefore p_{z}.BC  = p_{y}.BC
Now from equation (4) and (5), we find that
p_{x} = p_{y} = p_{z}
Thus the intensity of pressure at any point in a fluid, at rest, is the same in all direction.

  • 3

pascal law states that the pressure exerted is same at all directions if the fluid is at rest ...

 

let us take a prism shaped object abc-def  inside a container with fluid. the fluid exerts a pressure on the surface of object (i.e pa,pb,pc) of area corresponding to the forces fa ,fb ,fc on the surfaces of BECF,ADFC,ADEB. let the areas corresponding to the forces be Aa,Ab,Ac. Gravity is neglected becauze the object is very small.  

Fb sin (theta) = Fc                            ( by equilibrium)

Absin (theta)= Ac

Fb cos (theta) = Fa

Ab cos (theta) = Aa

therefore ,  Fb/Ab= Fc/Ac=Fa/Aa 

i.e Pb=Pc=Pa

hence proved that the pressures are equal in all directions 

THE FIGURE IS GIVEN IN TEXTBOOK PAGE NO-- 248 (2 PART )

 

 

Ab

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