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Calculating viscosity of a liquid with a falling object

  1. Jun 1, 2017 #1
    1. The problem statement, all variables and given/known data

    A small steel-bearing falls 25.0 cm in glycerol in 23.8 s and the same distance in castor oil in 15.1 s. The densities are for glycerol 1260 kg m−3 , for castor oil 961 kg m−3 , and for steel 7830 kg m−3 . The viscosity for glycerol is 1.490 Pa s. Calculate the viscosity for castor oil. All values are valid for 20 ◦C.


    2. Relevant equations
    I guess Viscosity=Density*Velocity where the velocity is equal to Distance/time.


    3. The attempt at a solution
    Thought I'd put the equation for the distance equal to each other and then rewrite it to get the viscosity of the castor oil, but I get the wrong value. It should be 0,988 Pas. I get the final equation to:
    Viscosity of Glycerol= (Density of Castor Oil*Viscosity of Castor*Time the steel bearing fell in the Glycerol)/(Density of Glycerol * time steel bearing fell in Castor oil)

    The equation has not regarded the denisty of the steel-bearing though, so that might be the problem but I don't see how I would get around to fit it in.
    It is very frustrating.
     
    Last edited by a moderator: Jun 1, 2017
  2. jcsd
  3. Jun 1, 2017 #2

    BvU

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    Well then, could it be that the net driving force for falling is a density difference ?
     
  4. Jun 1, 2017 #3

    Borek

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  5. Jun 2, 2017 #4
  6. Jun 2, 2017 #5

    BvU

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    Call it ##R##, cross your fingers and hope it divides out in the answer :smile:
     
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