Floating and sinking - Archimedes'
principle
You will be aware that some objects float in water and that others
sink. When you swim you feel that you are lighter and more buoyant.
The general concept that governs floating and sinking is called Archimedes'
principle.
Archimedes' principle
An object immersed in a liquid experiences an upward force, called
upthrust or buoyancy. This has been generally recognised for a long
time. It was the Greek mathematician and philosopher Archimedes (287-212
BC) who first put it into a general principle. His statement, now known
as Archimedes' principle, was that "when an object is immersed
in a liquid the upthrust is equal to the weight of liquid displaced
by the object". Thus, a rubber duck floats because its mass
is equalled by the water it displaces before it physically goes under
the surface. A solid lump of iron on the other hand will sink, because
the water cannot displace the mass of the iron before the iron sinks.
Why do steel ships float?
It's obvious that a piece of steel will sink in water, yet ships
made of steel float. There must be some other factor that comes into
play to explain this. Applying Archimedes' principle to the problem,
the upthrust of the water must be greater than the mass of the ship,
so somehow a great mass of water has to be displaced. This is done by
shaping the hull of the ship in such a way that as the ship sinks into
the water it displaces more and more liquid until a balance is reached
between the mass of water displaced and mass of the ship. This general
principle applies to any object made from a material that is more dense
than the liquid it is in.
Sinking ships
In the past, some European ships sank when they entered the tropics
for the first time. All was well when the cargo was put aboard in cold,
salty waters, but then the ship sank when it reached warmer, less salty
seas. The problem was that Archimedes' principle had not been taken
into account. When the ship was first loaded it floated because cold,
salty water has a relatively high density. This meant that less water
had to be displaced to equal the mass of the ship. As the ship steamed
to warmer, less salty waters, more water had to be displaced to maintain
equilibrium. The ship dropped lower in the water, sometimes dropping
below the waterline, and sinking. This was overcome by Samuel Plimsoll
who marked his ships with what became known as the Plimsoll Line.
How do submarines float and sink?
Using Archimedes' Principle, it is clear that a change in mass of an
object affects how much liquid has to be displaced. In submarines, this
is controlled by ballast tanks. When the tanks are empty, the submarine
has less mass and it floats like a normal ship. As water is allowed
into the tanks, the mass of the submarine increases, the downward gravitational
force on the submarine increases and the submarine begins to sink. Careful
balancing of the water ballast enables the craft to stay at any chosen
depth. The swim bladders of fish work in a similar manner.
What about balloons?
Archimedes' principle applies to any fluid so it works for a balloon
in air too. If a balloon is filled with a gas which is less dense than
air, it will float. In a hot air balloon the air trapped in the canopy
is less dense than the surrounding air so again the balloon will rise.
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