archimedes principle   Concepts covered: The Archimedes Principle | Objects Floating Freely   The Archimedes Principle Archimedes' principle is a law that explains buoyancy or up thrust. It states that When a body is completely or partially immersed in a fluid it experiences an up thrust, or an apparent loss in weight, which is equal to the weight of fluid displaced.   An object experiences up thrust due to the fact that the pressure exerted by a fluid on the lower surface of a body being greater than that on the top surface, since pressure increases with depth. Objects Floating Freely When an object is floating freely (i.e. neither sinking nor moving vertically upwards), then the upthrust must be fully supporting the object’s weight. We can say Upthrust on body = Weight of floating body By Archimedes’ principle, Upthrust on body = Weight of fluid displaced Therefore, Weight of floating body = Weight of fluid displaced This result, sometimes called the “principle of floatation”, is a special case of Archimedes’ principle and can be stated: A floating body displaces its own weight of fluid. If a body cannot do this, even when completely immersed, it sinks.   reference http://hsphysicslab.blogspot.com/2007/04/archimedes-principle-explained.html