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What does "specific
gravity" mean?

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The specific
gravity of an object

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is the density of
that object divided

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by the density of water.

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The density of water is 1,000
kilograms per meter cubed.

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For instance, the
density of gold

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is 19,300 kilograms
per meter cubed.

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So the specific gravity
of gold is 19.3.

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The density of ketchup is 1,400
kilograms per meter cubed.

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So the specific gravity
of ketchup up is 1.4.

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Note, there's no units
for specific gravity,

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because it's the ratio of one
density to another density.

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So the units cancel
each other out.

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OK, so why even bother
defining something

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called the specific gravity?

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Well, one really cool thing
about specific gravity

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is that, for
something that floats,

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the specific gravity
tells you the fraction

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of that object that
will be below the water

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while it's floating.

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For instance, say you let a cube
of wood with specific gravity

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0.2 float in water.

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Since the specific
gravity is 0.2,

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that means that 20% of the
total volume of this wood

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is going to be submerged below
the water while it's floating.

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If the cube of wood had a
specific gravity of 0.6,

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60% of the wood
would be submerged

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beneath the water's surface.

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Ice has a density of about
920 kilograms per meter cubed.

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That means ice has a
specific gravity of 0.92.

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And that's why 92% of
an iceberg's volume

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is actually
underneath the water.

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But what if we were
to use a cube that

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had a density of 2,700
kilograms per meter cubed?

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The specific gravity
would be 2.7,

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which means that
270% of this cube

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would be submerged
beneath the water.

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But you can't have more than
100% of an object submerged.

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Even if the object were to sink,
the maximum amount submerged

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would be 100%.

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So if the specific gravity of
an object is greater than 1,

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that object is going to sink
if placed freely in water.

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And it'll have exactly 100%
of its volume submerged.

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Usually when people
are referring

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to the specific gravity,
they're referring

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to the density of
the object divided

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by the density of water.

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But sometimes it's useful to
define the specific gravity

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with respect to a liquid
that's different from water.

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For instance, if you were
to let some wood of density

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638 kilograms per meter
cubed float in oil

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that has a density of 850
kilograms per meter cubed,

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you could still find the
percent of the wood that's

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submerged underneath the oil.

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Just use the density of oil
instead of the density of water

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in the specific gravity formula.

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If we do that, we'll find that
the wood does float in this oil

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with 75% of the wood's
volume submerged

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beneath the surface of the oil.

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