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.
