.....................................................................
Buoyancy and
Archimedes Principle
WHAT
IS BUOYANCY?
Buoyancy
is a force exerted by a liquid or gas that opposes an object's weight.
Buoyancy
can also be stated as the weight of displaced fluid. Pressure in a fluid
increases with depth as a result of the weight of the overlying fluid.
Thus
an object submerged in a fluid, experiences greater pressure at the bottom of
the fluid than at the top.
This
difference in pressure results in a net force that tends to accelerate an
object upwards.
The
magnitude of that force is proportional to the difference in the pressure
between the top and the bottom of the column, and is also equivalent to the
weight of the fluid that would otherwise occupy the column, i.e. the displaced
fluid.
It
is for that reason that an object whose density is greater than that of the
fluid that it is submerged in will sink.
Archimedes'
principle allows for the experimental determination of density by providing an
easy and accurate method for determining the volume of an irregularly shaped
object.
Archimedes'
principle is named after Archimedes of Syracuse, who first discovered this law
in 212 B.C.
Archimedes'
principle may be stated thus in terms of forces:
Any
object, wholly or partially immersed in a fluid, is buoyed up by a force equal
to the weight of the fluid displaced by the object.
For
objects, floating and sunken, and in gases as well as liquids, Archimedes' principle
may be stated in terms of forces:
Any
object, wholly or partially immersed in a fluid, is buoyed up by a force equal
to the weight of the fluid displaced by the object, with the clarifications
that for a sunken object the volume of displaced fluid is the volume of the
object, and for a floating object on a liquid, the weight of the displaced
liquid is the weight of the object.
Archimedes'
principle indicates that the upward buoyant
force that is exerted on a body immersed in a fluid, whether fully or partially
submerged, is equal to the weight of the fluid that the body displaces.
Notice
that the acting forces increase with the depth of the fluid.
The
resultant of all forces upward is called buoyancy and is equal to the weight of
the displaced fluid.
Archimedes
principle allows the buoyancy of an object partially or wholly immersed in a
liquid to be calculated.
The
downward force on the object is simply its weight. The upward, or buoyant,
force on the object is that stated by Archimedes' principle, above.
Thus
the net upward force on the object is the difference between the buoyant force
and its weight.
If
this net force is positive, the object floats; if negative, the object sinks;
and if zero, the object is neutrally buoyant - that is, it remains in place
without either rising or sinking.
In
simple words Archimedes principle states that when a body is partially or
completely immersed in a fluid, it experiences an apparent loss in weight which
is equal to the weight of the fluid displaced by the immersed part of the body.
Note: Archimedes' principle does not consider the surface
tension (capillarity) acting on the body but this additional force modifies
only the amount of fluid displaced, so the principle that Buoyancy
= weight of displaced fluid remains valid.
Consider
a cube immersed in a fluid, with its sides parallel to the direction of
gravity. Only the forces on the top and bottom faces of the cube will
contribute to buoyancy.
The
pressure difference between the bottom and the top face is directly
proportional to the height (difference in depth).
Multiplying
the pressure difference by the area of a face gives the net force on the cube - the
buoyancy, or the weight of the fluid
displaced.
FORMULAS
For
a fully submerged object, Archimedes' principle can be formulated as follows:
apparent
immersed weight = weight of object - weight of displaced fluid
density
of object/density of fluid = weight/(weight of displaced fluid)
density
of object/density of fluid =weight/(weight - apparent immersed weight)
The
"buoyancy force" on a submerged body is directed in
the opposite direction to gravity and is equal in magnitude to:
(density of fluid)(Volume of displace fluid)(acceleration)
When will an object float or balloon rise?
A
object will float in a fluid if the density of that object is less than the
density of the fluid.
Example:
If you drop wood into water, buoyancy will keep it afloat. Wood is less dense
than water.
Hot
air balloons rise into the air because the density of the air (warmer air)
inside the balloon is less dense than the air outside the balloon (cooler air).
The
balloon and the basket displaces a fluid that is heavier than the balloon and
the basket, so it has a buoyant force acting on the system. Balloons tend to
fly better in the morning, when the surrounding air is cool.
SAMPLE PROBLEM
A
steel block with a density of 7800 kg/m3 is suspended from a string in a beaker
of
water so that the block is completely submerged but not resting on the bottom. The block is a
cube with sides of 3 cm (0.03 m).
water so that the block is completely submerged but not resting on the bottom. The block is a
cube with sides of 3 cm (0.03 m).
Buoyant
force = weight of displaced water
= mass of water * acceleration due to gravity
= density of water * volume of cube * g
= 1x103 kg/m 3 * 27*10-6 m3 9.8 m/s2 = 2.65 *10-1 N = 0.265 N
= mass of water * acceleration due to gravity
= density of water * volume of cube * g
= 1x103 kg/m 3 * 27*10-6 m3 9.8 m/s2 = 2.65 *10-1 N = 0.265 N
weight
of block = mass g
W= mg = 2.106 x 10-1 kg x 9.8 m/s2 = 2.106x9.8 x 10-1 kg*m/s2 = 20.64 x 10-1 N
= 2.064 N kg*m/s2 = N (Newton)
W= mg = 2.106 x 10-1 kg x 9.8 m/s2 = 2.106x9.8 x 10-1 kg*m/s2 = 20.64 x 10-1 N
= 2.064 N kg*m/s2 = N (Newton)
Archimedes of Syracuse
ABOUT ARCHIMEDES --Archimedes was the son
of an astronomer. He had traveled to Alexandria, Egypt, a place of great
learning, where he studied the works of some other mathematicians, like Euclid
and Conon.
Archimedes helped his friend King Hiero II
by creating machines for the king's army. The pulley was one of these
inventions, but Archimedes thought the study of mathematics was the most
important thing he could do.
Archimedes wrote some books about
Mathematics, including On Floating Bodies.
Archimedes died during the Siege of
Syracuse when he was killed by a Roman soldier despite orders that he should
not be harmed.
No comments:
Post a Comment