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# Specific gravity

Video transcript

[MUSIC PLAYING THROUGHOUT] What does "specific
gravity" mean? The specific
gravity of an object is the density of
that object divided by the density of water. The density of water is 1,000
kilograms per meter cubed. For instance, the
density of gold is 19,300 kilograms
per meter cubed. So the specific gravity
of gold is 19.3. The density of ketchup is 1,400
kilograms per meter cubed. So the specific gravity
of ketchup up is 1.4. Note, there's no units
for specific gravity, because it's the ratio of one
density to another density. So the units cancel
each other out. OK, so why even bother
defining something called the specific gravity? Well, one really cool thing
about specific gravity is that, for
something that floats, the specific gravity
tells you the fraction of that object that
will be below the water while it's floating. For instance, say you let a cube
of wood with specific gravity 0.2 float in water. Since the specific
gravity is 0.2, that means that 20% of the
total volume of this wood is going to be submerged below
the water while it's floating. If the cube of wood had a
specific gravity of 0.6, 60% of the wood
would be submerged beneath the water's surface. Ice has a density of about
920 kilograms per meter cubed. That means ice has a
specific gravity of 0.92. And that's why 92% of
an iceberg's volume is actually
underneath the water. But what if we were
to use a cube that had a density of 2,700
kilograms per meter cubed? The specific gravity
would be 2.7, which means that
270% of this cube would be submerged
beneath the water. But you can't have more than
100% of an object submerged. Even if the object were to sink,
the maximum amount submerged would be 100%. So if the specific gravity of
an object is greater than 1, that object is going to sink
if placed freely in water. And it'll have exactly 100%
of its volume submerged. Usually when people
are referring to the specific gravity,
they're referring to the density of
the object divided by the density of water. But sometimes it's useful to
define the specific gravity with respect to a liquid
that's different from water. For instance, if you were
to let some wood of density 638 kilograms per meter
cubed float in oil that has a density of 850
kilograms per meter cubed, you could still find the
percent of the wood that's submerged underneath the oil. Just use the density of oil
instead of the density of water in the specific gravity formula. If we do that, we'll find that
the wood does float in this oil with 75% of the wood's
volume submerged beneath the surface of the oil. [MUSIC PLAYING]