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U.S. Navy Museum Cold War Gallery Lesson Plan
Snap, Crackle, Pop: Submarine Buoyancy, Compression, and Rotational Equilibrium
Developed by Bill Sanford, Nansemond Suffolk Academy, Suffolk, VA
2012 Naval Historical Foundation STEM-H Teacher Fellowship
 
 
  Instructional Goal:

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Explain and perform calculations regarding the buoyant force on a submarine, how the buoyant force on a submarine varies as its hull compresses, and how a submarine can maintain neutral buoyancy and rotational equilibrium by pumping water from tank to tank and on/off the boat. In an extension activity, students can calculate the deformation of the submarine due to water pressure at various depths.

Background:

Controlling Depth and Angle of the Sub:
  • A sub can adjust its density and thus the net vertical force it experiences by pumping water on or off. Its volume stays the same, but its mass changes, thus changing its density.
  • It can also control its vertical motion by changing some of its horizontal motion into vertical motion by angling its "planes".
  • It can also pump water from a tank in the front to an aft tank to help control the balance of the ship.
Does the buoyant force on an object change as the object is taken deeper in a fluid?
  • We assume it does not, since the buoyant force is determined by how much fluid the object displaces, which is determined by the object's volume, which we often assume to be constant
  • But...if the object is compressible, as it goes deeper into the fluid, it undergoes what we call Bulk Deformation, such that it takes up less space...occupies a smaller volume.
  • Compression: the decks of a submarine "float" on the hull so they do not buckle when the hull compresses. This can create creaking noises as the submarine proceeds deeper, unless the deck/hull junctions are properly greased.
Note: The following is not covered in the lesson, but is interesting. A special alloy is used for the hull. The trick for the metallurgist is to strike a compromise and to use the correct ratio of alloy elements to gain a hull plate that maximizes its resistance to pressure through high compression strength, but yields enough to allow the hull to bend instead of break.

Resources:

Submarine Buoyancy and Compression example


Effects of Compression as Object Goes Deeper
Submarine Buoyancy and Compression example 2

Click icon to download Activity Answers
Buoyancy Problems

Objective:

Practice and Reinforce concepts related to Fluid Pressure, primarily Buoyancy

Materials:

Activity 1 Worksheet:  You will complete this worksheet after viewing the video and reading the instructions below.

play videoWatch this video about how a submarine submerges:  This short segment from the 1954 U.S. Navy documentary "Take 'er Down" explains the use of air and water by submarines to submerge and surface. Source: Naval History and Heritage Command, UM-29.

Instructions:

Submarines are basically teardrop shaped, with a "sail": a rectangular protrusion on the top, from which the periscopes and antennas emerge. The image depicts a submarine that is moving to the right.

2 sets of planes for submarinesSubmarines have two sets of "planes" that can be angled so that the water flowing past them pushes them up or down. In this way, they are used to control the forward/aft angle of the submarine (its pitch). Similar to the wing flaps of an airplane, they are also used to change forward motion into vertical motion. One set is near the rear (aft) end of the submarine, or its stern. The other is either near the front (bow), or protruding from the sail.

Two ways to control the vertical position of a submarine:
  • Adjust its own density and thus the net vertical force it experiences by pumping water from its tanks into the ocean, or allowing some ocean water into its tanks.
  • Changing some of its horizontal motion into vertical motion by angling its "planes".
Does the buoyant force on an object change as the object is taken deeper in a fluid?
  • Normally we assume it does not, since the buoyant force is determined by how much fluid the object displaces, which is determined by the object's volume and the density of the fluid, which we often assume to be constant, but...
  • If and object is made of a compressible material, as it goes deeper into the fluid, it undergoes what we call Bulk Deformation, such that it takes up less space (occupies a smaller volume), so Yes, it experiences a weaker buoyant force as its volume decreases. A submarine's hull actually compresses (yes, it gets measurably smaller! not noticeably smaller, but measurably) due to the water pressure it experiences. Activity 3 of this lesson plan explores this concept farther.
    -- To prove this, some crews have tied a string tightly between the bulkheads (walls) while on surface. After diving to a deep depth, the string hangs loosely!
  • Also, although liquids are basically incompressible (you cannot make a defined amount of liquid take up less space by squeezing it, AKA its density does not change as you increase the pressure on it). Nevertheless, if the pressure is extreme, you can vary the density of a liquid.
  • Although pressure does have a small effect on the density of water, temperature has a larger effect, and the temperature of seawater drops sharply as you go deeper. Overall, the compression of the hull has a larger effect than either of these effects on the density of the seawater.
 
 

 

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