Hydrostatics Force
Pressure Force on Plane Surfaces:
where, = vertical depth of centroid of area from free liquid surface
A = Area of surface
where, I_{G} = Moment of inertia about centre of gravity of the body.
Pressure Force on curved surfaces: Horizontal component of the resultant hydrostatic force 'F_{x}' of curved surface may be computed by projecting the surface upon a vertical plane and multiplying the projected area by the pressure at its own centre of area.
Vertical component of force 'F_{y}' is equal to the weight of the liquid block lying above the curved surface upto free surface.
Resultant Force
Angle of line of action of resultant force with the horizontal is given by tan
Depth of center of pressure for some vertical plane surfaces
S.No  Surface  
1.  RECTANGLE  (h/2)  (2h/3) 
2.  TRAPEZIUM  
3.  TRIANGLE  (2h/3)  (3h/4) 
4.  CIRCLE  (d/2)  (5d/8) 
5.  SEMI CIRCLE  2D/3π  3πD/32 
6.  PARABOLA  (3h/5)
(2h/5)  (5h/7)
(4h/7) 
Archimedes principle and Force of buoyancy:
Stability of floating bodies: The stability of floating body is defined by relative position of metacentre (M) and centre of gravity (G).
A floating body is STABLE if, when it is displaced, it returns to equilibrium.
A floating body is UNSTABLE if, when it is displaced, it moves to a new equilibrium.
If M lies above G (GM = +ve) then it is stable equilibrium
If M lies below G (GM =  ve) then unstable equilibrium
If M coincides with G (GM = 0) then neutral equilibrium
GM = (I/V) (BG))
where GM = metacentric height.
I = moment of inertia about longitudinal axis of the plane at the level of free surface. ('I' is minimum moment of inertial of the plane.)
V = Volume of liquid displaced by the body.
BG = Distance between centroid of liquid displaced and centroid of the body.
For stable equilibrium of a circular cone floating with its apex downward having diameter 'd' and a vertical height 'h'. (The specific gravity of the cone is 'S'), The condition for stability is
Vortex Motion : A whirling mass of fluid is called vortex flow.
(i) Forced Vortex Flow:
When a fluid is rotated about a vertical axis at constant speed, such every particle of its has the same angular velocity, motion is known as the forced vortex.Forced vortex requires constant supply of external energy/torque.
h=ω^{2}r^{2}/2g
where h is height of paraboloid, and r is radius of paraboloid at top.
Volume of paraboloid = 1/2 πr^{2}h
The surface profile of forced vortex flow is parabolic.
Example of forced vortex flow is rotating cylinder & flow inside centrifugal pump.
(ii) Free Vortex Flow:
In this flow fluid mass rotates due to conservation of angular momentum. No external torque or energy is required. In free vortex flow Bernoulli's equation can be applied. The velocity profile is inversely proportional with the radius.
v r = constant
The point at the centre of rotation is called singular point, where velocity approaches to infinite.
Example of free vortex motion are whirling mass of liquid in wash basin, whirlpool in rivers etc.
2 videos122 docs55 tests

1. What is hydrostatic force? 
2. How is hydrostatic force calculated? 
3. What is the importance of hydrostatic force? 
4. How does the shape of an object affect the hydrostatic force? 
5. Can hydrostatic force be greater than the weight of an object? 
2 videos122 docs55 tests


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