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## Friday, October 5, 2012

### CBSE Class 9 - Science - Chapter 10: Flotation:Thrust and Pressure, Pressure

 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/m3
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 Nm-2 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 × 105 Nm-2 (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 m-3] g = gravitational acceleration [m s-2]
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.
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 cm3. If the density of water is 1 g cm−3, will the substance float or sink?

Ans: Density of substance: masss/volume = 50g/20cm3= 2.5g/cm3

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/cm3). Hence, it will sink in water.

The volume of water displaced by the packet =  the volume of the packet= 350 cm3

The mass of water displaced by the packet  = dxv = 1 g/cm3 x 350 cm =350g

Floating and Sinking: if density of object is more than the density of liquid object float unless sink
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