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### Course: Physics library > Unit 9

Lesson 1: Density and Pressure# Specific gravity

In this video David explains what specific gravity means. He also shows how to calculate the value for specific gravity and use it to determine the percent of an object that will be submerged while floating. Created by David SantoPietro.

## Want to join the conversation?

- What if the specific gravity of the object is the same as the specific gravity of the liquid?(82 votes)
- This will mean that it will not sink,but fully submerge under the liquid-hovering!(38 votes)

- why does the fraction submerge underwater is the specific gravity ?(17 votes)
- For a particle to be in Translational Equilibrium,

Net force acting on the particle should be 0.

The forces on a particle in a liquid are buoyant force and it's weight mg.

mass=Volume*density

=>M=Vp*Dp (Vp=Volume of the particle, Dp=Density of the particle)

=>mg=Vp*Dp*g

Buoyant force=Vpi*Df*g (Vpi=Volume of the particle immersed, Df=Density of the fluid)

=>Vpi*Df*g=Vp*Dp*g

=>(Vpi/Vp)=(Dp/Df)

U know that Dp/Df is the specific gravity of the particle and Vpi/Vp is the fraction submerged under the fluid.

So, the fraction submerge underwater is the specific gravity.

Hope this helps :)(40 votes)

- In the beginning of the video, what it the large p respresent?

Pascals??(3 votes)- I think its the lower case greek letter, rho, to describe density.(27 votes)

- What makes wood, and ice float in water?(7 votes)
- All objects have a buoyant force acting on them when they are in a fluid (gas or liquid) within a gravitational field. The gravity causes the fluid pressure to be greater the more downward you go. As a result a buoyant force pushing upwards is created from the larger pressure pushing on the bottom of the object versus the lower pressure pushing on the top. If the object has a larger downward force from gravity than the upward force from buoyancy, the object floats otherwise it will sink. All of this can be simplified to comparing the density of the object to the density of the fluid. If the object has higher density than the fluid, it will sink otherwise it will float.(16 votes)

- This might be a dumb question, but what's the name for the symbol to represent specific gravity? The one that looks like a p. Thanks!(5 votes)
- No dumb questions! The Greek lowercase letter rho ( ρ ) is usually used for density and specific gravity.(9 votes)

- Can an object with a specific gravity of 1 vary in how deep it is submerged or will it always just float directly beneath the surface of the fluid?(4 votes)
- Theoretically, the equilibrium is achieved with it floating just below de surface.(5 votes)

- Does specific gravity determine whether a human body will float? According to google the specific gravity of a human body is 1.01 so technically we should sink in water. What makes (some of us) float?(3 votes)
- Body fat has a specific density less than 1, so a person with more body fat will float. Also, the air in a person's lungs can act to make a person more buoyant.(4 votes)

- what if the liquid was made of a mixture of liquids with different densities(1 vote)
- the liquid mixture with the specific gravity lesser than one will form a layer on top of the liquid while the liquid mixture with greater specific gravity will form a layer on the bottom of the liquid(5 votes)

- Sometimes words bring more confusion, I just would like to know whether this one is one of these situations. So if we look at the word "gravity" we know that it is different on different planets. However, by definition, we said that the specific gravity is the ratio of the density of an object to the reference fluid density (e.g. water). Considering, that the density is the ratio of the mass to the volume, which are the same on every planet, does it mean that regardless of the planet you are on, the specific gravity for an object will be the same?(3 votes)
- Yes I believe that is correct. The ice in your ice water will float the same on Mars and Earth.(1 vote)

- so does this explain why people float in the Dead Sea but not in the regular ocean(2 votes)
- The extremally high concentration of salts increases the specific gravity so that its density is close to the density of the human body.(3 votes)

## 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]