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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.

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  • blobby green style avatar for user Madeleine Dionne
    What if the specific gravity of the object is the same as the specific gravity of the liquid?
    (79 votes)
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  • orange juice squid orange style avatar for user maite.cintron
    why does the fraction submerge underwater is the specific gravity ?
    (17 votes)
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    • leaf blue style avatar for user Sabbarish Govindarajan
      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 :)
      (38 votes)
  • aqualine ultimate style avatar for user talatisaumil
    In the beginning of the video, what it the large p respresent?
    Pascals??
    (3 votes)
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  • aqualine ultimate style avatar for user DAVID
    What makes wood, and ice float in water?
    (7 votes)
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    • leaf green style avatar for user Mark Zwald
      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.
      (14 votes)
  • duskpin ultimate style avatar for user Grace.M.McFadden
    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!
    (3 votes)
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  • aqualine seedling style avatar for user Summer A.
    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?
    (3 votes)
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  • blobby green style avatar for user Kay Gopinath
    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)
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  • purple pi purple style avatar for user Monika Hossain
    what if the liquid was made of a mixture of liquids with different densities
    (1 vote)
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  • duskpin ultimate style avatar for user bipulgupta5
    I was taught that density of ice is less than that of water...,also most of the ice is underwater, so why does the water level of oceans rise when ice melts due to global warming.., shouldn't the water level be decreasing because same mass of water occupies lesser volume when compared to that of same mass of ice?
    (1 vote)
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    • spunky sam red style avatar for user V_Keyd
      The main cause of the rise in the ocean levels is not because of the melting of ice that's floating in the seas and oceans. It's because as the temperature increases, the volume of the water above 4°C increases. This is called thermal expansion of water. Also, the rise in the global temperature is causing the glaciers and ice on land to melt which is adding more water to the oceans and adding to its rising level.
      (5 votes)
  • starky ultimate style avatar for user Gaurav Deevela
    Wait, so if the object is less denser than water it will float right? Then what if we put a paper in water? It will stay there for about 2 seconds and then starts to tear and sink a bit. Why?
    (2 votes)
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    • male robot hal style avatar for user Andrew M
      No, if the object displaces an amount of water whose weight is greater than the object's weight, then it will float.
      Why don't you try an experiment where you put a piece of paper in water and watch very carefully what happens. See if you can explain what's going on.
      (2 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]