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 sea water is about 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).