If the buoyant force is more than the object weighs, the object  will float. If the buoyant force is less than the object weighs,  the object will sink. For the object that sinks, its measurable  weight will be less by the weight of the displaced fluid.  Example: A 10-ft$^{3}$ object weighing 700 lb is placed in pure  water. Will the object float? If the object sinks, what is  its measurable weight in the submerged condition? If the  object floats, how many cubic feet of its volume is below  the water line?  The following experiment is illustrated in  Figure 5-38 . The  overflow can is filled to the spout with water. The heavy  metal cube is first weighed in still air and weighs 10 lb. It is  then weighed while completely submerged in the water and  it weighs 3 lb. The difference between the two weights is  the buoyant force of the water. As the cube is lowered into  the overflow can, the water is caught in the catch bucket.  The volume of water which overflows equals the volume  of the cube. The volume of irregular shaped objects can  also be measured by using this method. If this experiment  is performed carefully, the weight of the water displaced by  the metal cube exactly equals the buoyant force of the water,  which the scale shows to be 7 lb.  Archimedes (287–212 B.C.) performed similar experiments.  As a result, he discovered that the buoyant force which a fluid  exerts upon a submerged body is equal to the weight of the fluid  the body displaces. This statement is referred to as Archimedes’  principle. This principle applies to all fluids, gases as well as  liquids. Just as water exerts a buoyant force on submerged  objects, air exerts a buoyant force on objects submerged in it.  The object will sink because the buoyant force is less than the  object weighs. The difference between the buoyant force and  the object’s weight will be its measurable weight, or 76 lb.  The amount of buoyant force available to an object can be  calculated by using the following formula:  Two good examples of buoyancy are a helium filled  airship and a seaplane on floats. An airship is able to float  in the atmosphere and a seaplane is able to float on water.  That means both have more buoyant force than weight.  Figure 5-39  is a DeHavilland Twin Otter seaplane, with a  gross takeoff weight of 12,500 lb. At a minimum, the floats  on this airplane must be large enough to displace a weight  in water equal to the airplane’s weight. According to Title  14 of the Code of Federal Regulations (14 CFR) part 23, the  floats must be 80 percent larger than the minimum needed  to support the airplane. For this airplane, the necessary size  of the floats would be calculated as follows:  0.11 cu'  10 lb  3  7 0.11 cu'  Overflow can  Catch  bucket  Figure 5-39.  DeHavilland Twin Otter seaplane.  Figure 5-38.  Example of buoyancy.  5-27  Buoyant Force = Volume of Object × Density of Fluid Displaced  Divide the airplane weight by the density of water.  Multiply this volume by 80%.  12,500 ÷ 62.4 = 200.3 ft 3  200.3 × 80% = 160.2 ft 3  Buoyant Force = Volume of Object × Density of Fluid Displaced  = 10 (62.4)  = 624 lb 