When you are in a pool, and spread out your arms while floating on your back, your body floats rather well. However, when you curl your body into a ball while in water, you sink. These basic principles of logic help explain the principles of buoyancy.
Buoyancy, aka floatation, is dependent on how much water is pushing against you in a certain situation.
Buoyancy comes from the fact that fluid pressure increases with depth, and that the higher the fluid pressure, the more pressure is exerted in all directions (see Pascal's Principle). When in water, an object exerts a downwards force on the water while the water exerts an upward foce on the object.
If the total area of an object that makes contact with water is large enough, the object will float. The object must make room for its own volume by pushing aside or displacing an equal volume of liquid (see hydralic pressure principle). The solid body floats when it has displaced the same amount of water that equals its original weight (see Archimede's Principle).


Why does the block sink or float? 


Water has the density of 1 gram per cubic centimeter. The actual mass of an object subtracted from the apparent mass of the object submerged is equal to the density of water multiplied by the Volume of the object.
so by knowing the density of water and the volume of an object, you can find out the buoyancy force on an object.

 

 
Definitions:
Definitions:
Fluid Pressure: The pressure exerted by a static fluid depends only upon the depth of the fluid, the density of the fluid, and the acceleration of gravity. Fluid pressure does not depend on the total mass or total volume of a liquid.
Pascal's Principle: Pressure is transmitted undiminished in an enclosed static fluid. This makes it possible for a large multiplication of force, called the hydralic pressure principle.
The Hydralic Pressure Principle: The multiplication of force can be achieved by the application of liquid pressure. This allows for the lifting of a heavy load with a small force.
Density: The mass per unit volume of a substance under specified conditions of pressure and temperature.





