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Homework Help: Viscosity problems

  1. Sep 29, 2013 #1
    1. The problem statement, all variables and given/known data
    An auto lift consists of a 55.56 cm diameter ram which slides concentrically in a 55.58 cm diameter
    cylinder (see Figure 1). The cylinder engages a 2.8 m length of the ram at all times. The annular region
    between the ram and cylinder is filled with viscous oil. The combined mass of the automobile, auto rack
    and ram is 1750 kg. If the lift drops steadily at the speed of 0.25 m/s after being released, determine:
    a) the viscosity of the oil, in Pa.s,
    b) the shear stress at the surface of the inner cylinder.

    2. Relevant equations


    3. The attempt at a solution

    I can find the force which is the weight og ram+ram, also dy and dv. but i don't know how to find A. my friends told me that A = 2pi*2.8 put i don't think it was right because 2.8m is the length of the whole thing not diameter.
    Please help me. I need to give my class a presentation about this and it counts as a test.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 29, 2013 #2
    What is the surface area of the cylinder? What is the surface area of the ram? Is the difference really significant?
  4. Sep 29, 2013 #3
    the surface of the cyclinder is 4.89m^2 and the surface area for the ram is also 4.89m^2. and no i don't see any huge differences between those values. and when i tried to find the viscosity it give me the value of 1.4kg/ms or 14 P. I don't know if mine is correct or not :( could you check for me please
    Last edited: Sep 29, 2013
  5. Sep 29, 2013 #4
    That's what I get.
  6. Sep 29, 2013 #5
    thank you very much !
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