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Pressure in Fluids

Pressure is force per unit area and increases with depth in an incompressible fluid.

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USS Iowa

USS Iowa

License: CC BY-NC 3.0

Firing 50-caliber guns, during a 1984 demonstration, the USS Iowa was the only ship of its class to have served in the Atlantic Ocean during WWII. The USS Iowa has a total length of 270m and a displacement of 45,000 tons. Between 1942 and 1990, the USS Iowa received nine battle stars for service in WWII and the Korean War.

Amazing But True

  • According to Archimedes Principle, an object placed in a fluid will experience and upward force that is equal to the weight of the fluid that is displaced by the object. For a ship that weighs 45,000 tons to float, it will need to displace 45,000 tons of water. Since a majority of the interior of any boat is mostly air, the combination of the boat’s material and air make it very light compared to the average density of water. An important point to remember is that the act of floating has more to do with the pressure than weight, which explains why heavy objects that float in water have a very large surface area that is in contact with the water.
  • To help determine whether a loaded boat will float, most commercial ships also have little known marks called Plimsoll marks to help determine how much cargo the ship can handle. These marks represent where the waterline should be for either fresh water or salt water voyages. These marks allow for proper loading of the ships to ensure that the ship does no float dangerously low wherever it may take its cargo.
  • Read about how cruise ships are constructed: http://adventure.howstuffworks.com/cruise-ship2.htm

Show What You Learned?

Using the information provided above, answer the following questions.

  1. An orange is completely submerged in a body of water. If the apply is moved to a point that is 10 m below the point it was initially submerged how much extra force is needed to hold it in this new position?
  2. The pressure at the bottom of a glass of unknown liquid of density 1000 kg/m3 is given as \begin{align*}P_o\end{align*}. If all the liquid is removed and replaced with the same total amount of a different liquid that has half the density, what would you expect of the pressure at the bottom of the glass?
  3. Which of the two exerts a great pressure: a 100 lb woman standing on hears of diameter 0.5 in or a 1000 lb elephant with a foot diameter of approximately 17.7 in?

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  1. [1]^ License: CC BY-NC 3.0

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