class 9 - floatation - sum up

GRAVITATION ( FLOATATAION)

Thrust and Pressure

·        Thrust is defined as the force acting perpendicular to a surface.

·        The S.I. unit of thrust is NEWTON (N).

·        Thrust acting on a unit surface area is called pressure.

·        Pressure = force / area

P = F/A

·        S.I unit of pressure is Pascal (Pa) or Newton/ meter².

·        Pressure exerted on a surface is said to be 1 Pa if a thrust of 1 N is exerted on an area of 1 m².

·        The pressure exerted depends upon two factors-

1.   Force applied.

2.   Area covered.

·        The same force can produce different pressure depending upon the area over which it acts.

·        Pressure is inversely proportional to area on which thrust is exerted. I.e. if area is more, pressure reduces. Similarly, if area is less, pressure increases.

Buoyancy

·        All liquids and gases are called fluids.

·        When a body is partially or fully immersed in a fluid, it experiences an upward force due to the fluid displaced by it. This upward force is called buoyant force or Upthrust.

·        The magnitude of the buoyant force is independent of the shape of the container but it depends upon the other two factors-

1.   Volume of the object immersed in liquid –

Volume of the body or object = volume of the liquid displaced.

More volume of displaced water = more weight of displaced water

More weight of displaced water = more upthrust.

2.   Density of the fluid –

Greater is the density of the fluid, more is the upthrust exerted by fluid on the object.

·        Since the upthrust acts on body, thus body experiences an apparent loss in weight. i.e.,

Apparent weight = actual weight – upthrust.

Thus, a body appears light in water as compared to air.

·        Maximum upthrust acts when body is completely immersed in water. Thus maximum loss in weight takes place and body is lightest at that point.

Principle of floatation

·        It states that if the body of weight ‘W’ is immersed in a fluid, such that the upthrust ‘U’ acts on it, then

a)   A body sinks if W > U

b)   A body floats when W < U

c)   A body floats with its top surface is completely immersed in fluid when W = U.

Density

·        Density of a body is defined as mass per unit volume.

Ƿ = M / V

·        S.I. unit of density is Kg/m³.

·        Density of water is 1000 Kg/m³ or 1 g/cm³.

·        Density of object and fluid also decides whether the object will sink or float.

·        When density of fluid is more, then more upthrust is acting on the object and object floats. But if density of object is more than fluid then the object will sink.

Archimedes’s principle

·        It states that when a body is partially or fully immersed in fluid, it experiences an upthrust which is equal to the weight of the fluid displaced by it.

Upthrust = weight of the displaced fluid

                   = V Ƿ g

Where, V = volume displaced

               Ƿ = density of fluid

               g= acceleration due to gravity.

·        This principle has many applications. Like it is applied practically to design ships and submarines. It is also used to make lactometers (device used to measure purity of milk) and hydrometers ( device used to measure density of water).

Relative density

·        The ratio of density of a substance to the density of water is called its relative density.

Relative density = density of substance / density of water

·        Since it is a ratio, it has no units.

·        When the relative density of a substance is greater than 1, it sinks.

·        When the relative density of a substance is less than 1, it floats.

·        Relative density expresses heaviness of a substance in comparison of water. E.g. by saying relative density of silver is 10.8, we mean that silver is 10.8 times heavy as an equal volume of water.