WEBVTT - Why can boats float on water?

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<v Speaker 1>Welcome to brain Stuff from house stuff Works dot com

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<v Speaker 1>where smart Happens. Hi Am Marshall Brain with today's question,

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<v Speaker 1>why can boats made out of steel float on water

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<v Speaker 1>when a steel bar sinks? The standard definition of floating

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<v Speaker 1>was first recorded by Archimedes and goes something like this.

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<v Speaker 1>An object in a fluid experiences and upward force equal

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<v Speaker 1>to the weight of the fluid displaced by the object.

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<v Speaker 1>So if a boat weighs a thousand pounds, it will

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<v Speaker 1>sink into the water until it has displaced a thousand

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<v Speaker 1>pounds of water. Provided that the boat displaces a thousand

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<v Speaker 1>pounds of water before the whole thing has submerged, the

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<v Speaker 1>boat floats. It's not very hard to shape a boat

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<v Speaker 1>so that the weight of the boat has been displaced

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<v Speaker 1>before the boat is completely underwater. The reason it's so

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<v Speaker 1>easy is that a good portion of the interior of

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<v Speaker 1>any boat is air, unlike a cube of steel, which

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<v Speaker 1>is solid steel. Throughout the average density of a boat.

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<v Speaker 1>The combination of the steel and the air is very

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<v Speaker 1>light compared to the average density of water, so very

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<v Speaker 1>little of the boat actually has to submerge into the

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<v Speaker 1>water before it has displaced the weight of the boat.

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<v Speaker 1>The next question to ask involves floating itself. How do

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<v Speaker 1>the water molecules know when a thousand pounds of them

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<v Speaker 1>have gotten out of the way. It turns out that

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<v Speaker 1>the actual act of floating has to do with pressure

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<v Speaker 1>rather than weight. If you take a column of water

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<v Speaker 1>one inch square and one foot tall, it weighs about

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<v Speaker 1>point four four pounds depending on the temperature of the water.

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<v Speaker 1>That means that a one foot high column of water

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<v Speaker 1>exerts point four or four pounds per square inch. If

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<v Speaker 1>you were to submerge a box with a pressure gauge

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<v Speaker 1>attached into the water, then the pressure gauge would measure

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<v Speaker 1>the pressure of the water at that submerged depth. If

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<v Speaker 1>you were to submerge this box one foot into the water,

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<v Speaker 1>the gauge would read point four four p s I.

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<v Speaker 1>What this means is that the bottom of the box

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<v Speaker 1>has an upward force being applied to it by that pressure.

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<v Speaker 1>So if the boxes one ft square and it's submerged

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<v Speaker 1>one ft deep, the bottom of the box is being

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<v Speaker 1>pushed up by the water pressure of twelve by twelve

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<v Speaker 1>inches times point four four pounds per square inch or

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<v Speaker 1>sixty two pounds. This just happens to exactly equal the

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<v Speaker 1>weight of a cubic foot of water that's been displaced.

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<v Speaker 1>It's this upward water pressure pushing on the bottom of

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<v Speaker 1>the boat that is causing the boat to float. Each

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<v Speaker 1>square inch of the boat that's underwater has water pressure

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<v Speaker 1>pushing it upward, and this combined pressure floats the boat.

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