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Buoyancy

Buoyancy is defined as a vertical force exerted on an object due to the weight of a fluid that the object displaces. This force was first discussed by Archimedes in 212 B.C.

Metal anchors sink to the bottom of a harbor. Yet ships may be made of metal and float. First let’s discuss a floating block of wood. When a block of wood is first placed on a still surface of water, it pushes some water out of the way, so that the block sinks a distance down into the water. The block “displaces some water” is a more technical way to say that the block “pushes some water out of the way”. That block of wood makes a depression in what had been a flat surface of water. If you lift the block out of the water, the water that had been displaced out of its normal place rushes back into the empty depression.

That displaced water provides the force that allowed the wood to float. When the block is floating, the displaced water is trying to reclaim its original location. The weight of the displaced water is the source that provides the ability of the displaced water to push up on the block. The weight of the displaced water is exactly equal to the weight of the block. A state of equilibrium is reached when the weight of the block and the weight of the displaced water are equal. The displaced water is pushing up with a force equal to its weight . . . and the weight of the block. The image of the displaced water using its weight to try to reclaim its original position is offered as a way to understand this concept.

If you have several blocks that are all cut to identical dimensions, but each is made of a different type of wood, you would probably find that most float with different percentages above the surface of the water. The heavier blocks sink deeper into the water before the forces reach equilibrium -- even though they have equal dimensions as the other blocks. The heaviest block is made of wood with the highest density, and because of that it has the smallest percentage staying above the water’s surface.

Now we can talk about metal boats and ships. The ships are made of heavy metal, but the ship is shaped so that most of the space inside the hull is a space full of air The total weight is much less than it would be if spaces inside the hull were all metal. Again, it’s a matter of the weight of the water displaced having the same weight of the ship. The US Navy Battleship Wisconsin displaces 57.540 tons fully loaded. That means the ship weighs 57.540 tons, and so does the water it displaces while floating. The Wisconsin sinks deep into the water to displace this much water. It requires water at least 38 feet deep. . The space inside the hull is not required to have a roof to be buoyant. Canoes are often made of aluminum and are open to the sky, but they float -- as long as they stay right-side up.

copyright: Steve Johnson, sojphysics@gmail.com

Steve Johnson is an electrical engineer. He earned a B. Sc. in Physics, and MS Electrical Engineering. Steve welcomes your questions.

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