Lab 8 Forces on Submerged Objects.docx

Lab 8 Forces on Submerged Objects.docx - Lab 8 Lab 8 Forces...

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Lab 8 Lab 8: Forces on Submerged Objects A. Background Information When an object is submerged in a fluid (liquid or gas), it will experience a force due to the pressure exerted by the fluid on the object. The amount of force depends on the density of the fluid and the depth to which the object is submerged. Understanding the amount of force exerted by a fluid on an object is very important, especially when designing hydraulic systems, submergible vehicles, or aircraft. The problem with the forces felt by submerged objects does not lie directly with the force exerted by the liquid, but instead because of differences in pressure inside and outside of an object. When standing at sea level, a person is actually experiencing a pressure of 101300 N/m 2 or 14.5 lbs/in 2 . This amount of force should easily crush you, but because of the way our bodies are designed, they push out with an equal amount of force. However, if you were to dive into a pool and swim down 33 feet (around 10m), you would double the amount of pressure your body is experiencing. When a submarine travels underwater, it experiences a sizable pressure on the outer hull, but the pressure on the inner hull needs to be maintained at around sea level so that the human crew can survive. It is this difference in pressure that needs to be taken into account when designing the submarine, especially at weak points such as external doorways and hatches. A similar problem is encountered by aircraft, except reversed. When an aircraft is flying at cruising altitude, the pressure outside the cabin is significantly lower than the pressure maintained inside the cabin to allow the passengers and crew to survive. This again causes a pressure difference which must be taken into account so that windows and doors don’t blow out. This is also partially responsible for the effect seen in movies when a door is opened on an aircraft and people get sucked out. It is the swift movement of the air inside the aircraft out through the newly opened passage attempting to equalize the pressure that throws people and objects out of the plane. B. Understand the Process The force that acts on a horizontal object with a surface area A at a depth of y in a liquid of density ρ can be calculated as: Force = (P o + ρgy)*A where P o is standard atmospheric pressure (101353 N/m 2 ) and g is acceleration due to gravity (9.81 m/s 2 ). The term (P o + ρgy) adjusts the pressure for the distance below the surface of the fluid due to the weight of the fluid above the object. However, if the object is vertical, or if the area of the object is not able to be calculated easily, a slight modification needs to be made to the equation above, as the pressure will differ from the top of the plate to the bottom. Assume we have the situation shown in figure 1 with an oddly shaped plate submerged in liquid.
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  • Spring '14
  • Fernandez

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