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Fluids Question

  1. Jun 10, 2005 #1


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    An object weighing 324 N in air is immersed in water after being tied to a string connected to a balance. The scale now reads 290 N. Immersed in oil, the object weighs 303 N. Calculate the density of the object.

    B)Calculate the density of the oil.

    Part A:

    The mass of the object is 33.027 kg
    and the mass when submerged in water is 29.561 kg.

    This means the volume would be 3.466 because the volume of the object is the volume of the displaced water.

    So, 33.027 kg/ 3.466 kg =9.5288 and the answer is wrong.
    What am i doing wrong.

    As for B i dont know how to tackle it
  2. jcsd
  3. Jun 10, 2005 #2

    Doc Al

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    Staff: Mentor

    The mass of the object doesn't change when submerged. What forces act on the object? Figure out what the buoyant force must equal and use that to figure the volume of the object.
  4. Jun 10, 2005 #3


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    Staff Emeritus
    Science Advisor

    I wouldn't word it that way! The mass of an object doesn't change when it is submerged in water- it's weight does because of an upward bouyancy force. What is true here is that there is an upward bouyancy force of 324- 290 = 34 N. That would be equivalent to 34/9.81= 3.466 kg of water or, since water has a density of 1 g per cc, displacing 3466 cm3 of water. The object has a volume of 3.466 kiloliters.
    I would strongly advise you to be careful about your units! The mass of the object is 324/9.81= 33.027 kg so its density is 33027 g/3466 cc= 9.5288 grams per cubic centimeter. Why do you say the answer is wrong?

    The bouyancy force due to the oil is 324-303= 21 N so the oil displaced has a mass of 21/9.81= 2.141 kg= 2141 grams. The volume of oil displaced is the volume of the object, 3466 cubic centimeters.

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