Speaker 1: Welcome to brain Stuff from house stuff Works dot com
Speaker 1: where smart Happens. Hi Am Marshall Brain with today's question,
Speaker 1: why can boats made out of steel float on water
Speaker 1: when a steel bar sinks? The standard definition of floating
Speaker 1: was first recorded by Archimedes and goes something like this.
Speaker 1: An object in a fluid experiences and upward force equal
Speaker 1: to the weight of the fluid displaced by the object.
Speaker 1: So if a boat weighs a thousand pounds, it will
Speaker 1: sink into the water until it has displaced a thousand
Speaker 1: pounds of water. Provided that the boat displaces a thousand
Speaker 1: pounds of water before the whole thing has submerged, the
Speaker 1: boat floats. It's not very hard to shape a boat
Speaker 1: so that the weight of the boat has been displaced
Speaker 1: before the boat is completely underwater. The reason it's so
Speaker 1: easy is that a good portion of the interior of
Speaker 1: any boat is air, unlike a cube of steel, which
Speaker 1: is solid steel. Throughout the average density of a boat.
Speaker 1: The combination of the steel and the air is very
Speaker 1: light compared to the average density of water, so very
Speaker 1: little of the boat actually has to submerge into the
Speaker 1: water before it has displaced the weight of the boat.
Speaker 1: The next question to ask involves floating itself. How do
Speaker 1: the water molecules know when a thousand pounds of them
Speaker 1: have gotten out of the way. It turns out that
Speaker 1: the actual act of floating has to do with pressure
Speaker 1: rather than weight. If you take a column of water
Speaker 1: one inch square and one foot tall, it weighs about
Speaker 1: point four four pounds depending on the temperature of the water.
Speaker 1: That means that a one foot high column of water
Speaker 1: exerts point four or four pounds per square inch. If
Speaker 1: you were to submerge a box with a pressure gauge
Speaker 1: attached into the water, then the pressure gauge would measure
Speaker 1: the pressure of the water at that submerged depth. If
Speaker 1: you were to submerge this box one foot into the water,
Speaker 1: the gauge would read point four four p s I.
Speaker 1: What this means is that the bottom of the box
Speaker 1: has an upward force being applied to it by that pressure.
Speaker 1: So if the boxes one ft square and it's submerged
Speaker 1: one ft deep, the bottom of the box is being
Speaker 1: pushed up by the water pressure of twelve by twelve
Speaker 1: inches times point four four pounds per square inch or
Speaker 1: sixty two pounds. This just happens to exactly equal the
Speaker 1: weight of a cubic foot of water that's been displaced.
Speaker 1: It's this upward water pressure pushing on the bottom of
Speaker 1: the boat that is causing the boat to float. Each
Speaker 1: square inch of the boat that's underwater has water pressure
Speaker 1: pushing it upward, and this combined pressure floats the boat.
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