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buoyancy





illustration of buoyancy
The upward thrust exerted upon a body immersed in a fluid. Buoyancy is equal to the weight of the fluid displaced (see Archimedes' principle). Thus a body weighs less when weighed in water, the apparent loss in weight being equal to the weight of the water displaced. For accurate weighing of bodies in air, a small allowance has to be made to correct for the buoyancy of the body.


Floating and sinking

In order to float, an object must be able to push aside its own weight of water. The pictures on the right show that any 'block' of water floats because its weight acting downward is balanced by an upward pressure. If you took away the block of water, the forces that had been holding it up would be available to support some other object of the same weight. They could easily support an empty gasoline can because the weight of the metal can plus the air inside would be much less than a similar volume of water. But they could not support the same can filled with water, for the weight of the can plus the water inside would be greater than a similar volume of water. In other words, the upward force acting upon an object in water equals the weight of the water pushed aside, so the object must displace its own weight in water if it is to float. This is the principle put forward by Archimedes, more than 2,000 years ago.

A lump of metal will not float but the same piece of metal made into thin plates and shaped so that it will push aside a much larger amount of water (like the gas can) will float. A ship is just a specially constructed tank, displacing enough water to float, even though it has to carry a cargo as well as its own weight. If, through some mishap, the ship begins to fill with water, it will become just like the water-filled gas can. Through the added weight of the rising water the ship will sink lower and lower in order to displace more water to balance its increasing weight. Finally, when the weight of the ship and the water in it becomes more than the weight of the water it can displace, it will sink.

Steel ships, holed below the water line by an exploding torpedo or mine in time of war, have been known to fill with water and sink in a matter of minutes. Wooden ships, on the other hand, have been known to remain afloat for a long time when holed and filled with water. The reason is that a log of timber normally contains tiny air-filled cavities between its fibers. Because of this, it is fairly bulky in proportion to its weight and, like the empty gas can, is much lighter than a similar volume of water, When it is filled with water the weight of the ship plus the water inside is still less than the weight of a similar volume. It will just float lower in the water, displacing more water to balance its increased weight, just as a gas can filled with water will float lower than an empty can. Eventually the ship will sink because water gradually seeps in between the fibers of the wood and replaces the air held in the cavities until the weight of the water-filled and water-logged ship becomes greater than a similar amount of water.

Some kinds of timber, such as ironwood and ebony, will not float at all. This is because the fibers in them are packed so closely that a log of the timber weighs more than a similar volume of water. On the other hand, balsa wood floats with very little of its volume submerged. It is so light, and its fibers so loosely packed, that it only needs to sink a little way into the water to push aside its own weight of water.


Related category

   • CLASSICAL MECHANICS