Its all in one word Buoyancy. Buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column
of fluid, pressure 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 due to the column of fluid than it experience at the top
of the column 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. For this reason,
an object whose density is greater than that of the fluid in which it is
submerged tends to sink. If the object is either less dense than the liquid,
the force can keep the object afloat or even raise it to the top of the fluid
column. This can occur only in a reference frame which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward".
What is the magnitude of the upward force by the fluid (Buoyancy)?
In static fluid, the net upward buoyancy force
is equal to the magnitude of the weight of fluid displaced by the body as
explained by Archimedes' principle. That is Buoyancy is commensurate to the weight of
the fluid that would otherwise occupy the column, i.e. the displaced fluid. The center of buoyancy of an object placed in
a fluid is the centroid of the displaced volume of fluid.
Buoyancy
= weight of displaced fluid.
In simple terms, the principle
states that the buoyancy force on an object is going to be equal to the weight
of the fluid displaced by the object, or the density of the fluid multiplied by
the submerged volume times the gravitational acceleration, g. Thus, among
completely submerged objects with equal masses, objects with greater volume have
greater buoyancy. This is also known as upthrust.
'Buoyancy force = weight of object in empty space −
weight of object immersed in fluid'
This is the reason why logs of woods
and even some other material floats when place on water while a metal coin
cannot. The weight of the liquid displaced by the coin is not enough to offset
its weight that is pulling it downward into the liquid, thus the coin sinks. If
that metal coin is, however, place in a mercury column, it would not sink
completely before it will have offset its weight due to the weight of the
displaced mercury, as such, it will float.
A metallic coin (one
British pound coin) floats in mercury due to the buoyancy force upon it and
appears to float higher because of the surface tension of the mercury.
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.
How does large metallic ship float?
The key lies in the shape of these
big mammothious giant. Ship as we know have large amount of its body intended
to be submerged in water to increase its buoyancy. It’s buoyancy is larger than
it’s weight. Really? Yes really. This is because the large ship we see are
largely made of open space in the form of cabins, engine rooms and many other
utility spaces, thus making the ship to have the configuration of an empty box.
The weight of the ship is thus not commensurate with its enormous size. Hence,
its weight pulling it downward is far less than the weight of the amount of
water it must displace to sink completely. The
fraction of the size below the water is commensurate to the volume off water it
must displace to offset its weight. That
is why an empty metal tank also seem to float until it becomes
filled with
water.
Reference : wikipedea
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