CBSE Class 9  Science  Chapter
10: FLOTATION

Thrust and Pressure, Pressure in Fluids,
Buoyancy

Fluid: Fluid is a substance that flows. For example, liquids and gases. They take the shape of container in which they are stored.
The
study of fluids at rest is known as hydrostatic or fluid static’s. The study of fluids in motion is termed as hydrodynamics.
THRUST:
The normal force acting on a unit surface area of the fluid is called
thrust
Density: The mass per unit volume of a
substance is called density.
D
= mass /volume Þ Mass of substance = d x v
 (i)
SI unit is kg/m^{3}
The
density of water (1 g cm^{−3})
PRESSURE IN A FLUID
The
pressure (P) is defined at the magnitude of the normal force acting on a unit
surface area of the fluid.
P
= F/A .
Hence pressure is in directly proportional to Area of
contact.
For
example a nail or a knife have pointed and sharp edge to reduce area of contact
to apply large pressure.
In C.G.S. system, unit of pressure is dyne/cm2. S.I. unit of pressure is Nm2 or Pascal (PA). The unit of pressure, Pascal (Pa) has been named in the honour of great French scientist and philosopher Blasie Pascal.
Another
unit of pressure is atmosphere (atm).
1
atm = 1.013 × 10^{5} Nm2 (or Pa)
1
atm. or one atmosphere is the pressure exerted by our atmosphere on earth
surface due to the weight of atmosphere.
The
pressure is a scalar quantity . This is because pressure is transmitted equally
in all directions when force is applied, which shows that a definite direction
is not associated with pressure.
Pressure Exerted by a Liquid Column
Weight of liquid inside the vessel = volume ×
density of liquid × acceleration due to gravity
Thrust
of liquid = weight of liquid = mass x gravitational acceleration= d x v x
g from (i)
Buoyant
force = Weight of fluid displaced = Vρg
Where
V = volume of fluid displaced [m3] ρ = density of fluid [kg m3] g =
gravitational acceleration [m s2]
Remember:
(i) The liquid at rest exerts equal
pressure in all direction s at a point inside the liquid.
(ii) Total pressure at a depth h below the
liquid surface = P + hrg where P =
atmospheric pressure.
(i ii) Liquid pressure is independent of
shape of the liquid surface, but depends upon the height of liquid
column.
column.
BUOYANCY AND FORCE OF
BUOYANCY
When
a body is immersed fully or partially in a fluid, it displaced the fluid whose
volume is equal to the volume of the body immersed in the fluid. This displaced
fluid exerts an upward force on the body called the force of buoyancy.
This
tendency of the displaced fluid to exert an upward force) is called buoyancy. OR,
The tendency of an object to float in a liquid or the
power of liquid to make an object float is called buoyancy
Up thrust or buoyant
force depends on:
(i) The size or volume of the body immersed in a liquid.
(ii)
The density of the liquid in which the body is immersed.
Quest Time (Answer these questions)
Q. What are the factors the buoyant force depends on?
Answer: 1. Volume of the object immersed in the liquid. 2. Density
of the liquid.
Q. Name the factors the buoyant force does not depend on?
Ans: Buoyant force does not change with depth.
Q .In what direction does the buoyant force on an object immersed in
a liquid act?
Ans: in the upward direction
Q. Why
does a block of plastic released under water come up to the surface of water?
Ans: It is because the upward buoyant force acting on
plastic block is greater than the downward gravitational force
Q. The volume of 50 g of a substance is 20 cm^{3}.
If the density of water is 1 g cm^{−3}, will the substance float or
sink?
Ans: Density of substance: masss/volume =
50g/20cm^{3}= 2.5g/cm^{3}
The density of the substance is more than the density of water (1
g cm−3). Hence, the substance will sink in water.
Q. The volume of a 500 g sealed packet is 350 cm3.
Will the packet float or sink in water if the density of water is 1 g cm−3?
What will be the mass of the water displaced by this packet?
Ans: Density of substance: masss/volume =500/350
=1.428
The density of the substance is more than the
density of water (1 g/cm^{3}). Hence, it will sink in water.
The volume of water displaced by the packet
= the volume of the packet= 350 cm^{3}
The mass of water displaced by the packet =
dxv = 1 g/cm^{3 }x 350 cm^{3 }=350g
Floating and Sinking: if density of object is
more than the density of liquid object float unless sink
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