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Measuring the density of an aircraft carrier

  1. Mar 30, 2012 #1
    An object floats if it is less dense than water. I'm assuming that an aircraft carrier can float because there are several rooms in it with air. What happens when the door on the upper most floor is opened? Does the air inside that room still contribute to the total average density of the ship? How exactly do you calculate the density of an aircraft carrier. Where do you draw the line when calculating its volume? If a door is open does the room to that door count towards the ship's volume irregardless of whether or not the door is open or not?
     
  2. jcsd
  3. Mar 30, 2012 #2
    The room contains air whether or not the door is open. Every atom and molecule aboard her contributes to her mass, the totality of which should remain less than the total mass of the water she's displacing, giving her buoyancy. If she gets torpedoed, and enough of her watertight compartments are open to the sea, the volume of air within her will rapidly be displaced by the denser water, and she will achieve decreasing buoyancy, followed by neutral buoyancy when the mass of the water plus steel plus everything else plus air she contains is equal to the mass of the volume of water she displaces, followed by negative buoyancy when the mass of the water plus steel plus everything else plus air she contains is greater than that of the volume of water she displaces, and she sinks.
     
  4. Mar 30, 2012 #3
    Include the air below the water line. Here is why...

    Think about a simplified case of a glass floating in a sink. The forces pushing the glass down are the weight of the glass and air pressure. Air pressure is the weight of the column of air above the glass. The force pushing it up is the water pressure below the water line. Part of the water pressure comes from the air pressure at the waters surface. The air pressure pushing down on the bottom of the glass however is slightly greater because it has a slightly taller column of air pushing down on it because the bottom of the glass is lower then the water line.

    In other words the weight of the air from the water line up can be neglected because the same weight of air is pushing down on the water so the net effect on buoyancy is zero. The weight of the air below the water line however, must be considered because it is balanced against an equal volume of water.
     
  5. Mar 30, 2012 #4

    Drakkith

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    Staff: Mentor

    Whether the door is open or closed doesn't matter, as the air pressure inside and outside the ship are pretty much equal. Also, I don't think it's really the density of the ship that matters, it is the total weight of the ship compared to the total weight of water it displaces. For example, if you put 500,000 tons of styrofoam on a platform on the ship, you could very well sink the ship! (At least until the foam began to submerge, but by then the ship is already submerged)
     
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