BUOYANT FORCE - ARCHIMEDE'S
PRINCIPLE

When a rigid object is submerged in a fluid (completely or partially), there
exits an upward force on the object that is equal to the weight of the fluid
that is displaced by the object.

Explanation: When the object is removed, the volume that the object
occupied will fill with fluid. This volume of fluid must be supported by the
pressure of the surrounding liquid since a fluid can not support itself. When no
object is present, the net upward force on this volume of fluid must equal to
its weight, i.e. the weight of the fluid displaced. When the object is present,
this same upward force will act on the object.

The net force on the object is given by,

* When the density of the object is less than that of the fluid, the net
force will be upwards and the object will rise. A Helium filled balloon is a
good example.

QT Movie Demonstration of Archimedes' Principle (2.3M)

Floating Objects:
When an
object is floating, the net force on it will be zero. This happens when the
volume of the object submerged displaces an amount of liquid whose weight is
equal to the weight of the object. A ship made of steel can float because it can
displace more water than it weighs.

Thus the fraction of a floating object that is submerged stand in ratio to
the density of the object to the density of the fluid. For objects with a
vertical wall around the outside edge like a rectangle, we can reduce the above
equation even farther,

Example 1: Since the density of ice is 917 kg/m3 while that
of pure water is 1024 kg/m3 one can see that the statement "your only
seeing the tip of the iceberg" really does mean that 917/1024 = 89.6% of an
iceberg is under water. The ice you see above the water is only about 10% of the
ice that there.

Example 2: Humans are made mostly of water and are thus very
close to being neutrally buoyant. However, if you take a large breath and hold
it you can become positively buoyant since your overall volume increases thus
lowering you overall density. Fat cells are less dense than water so people who
carry a lot of extra fat can float more easily.

Example 3: A can of Diet Coke will float while a can of Classic Coke
will not float because Classic Coke has 11 teaspoons of sugar which are not in a
can of Diet Coke. Sugar is more dense than water. See Floating Diet Coke QT Movie (978K).