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Hydraulic piston break system problem

  1. Dec 3, 2012 #1
    1. The problem statement, all variables and given/known data

    The figure below shows the essential parts of a hydraulic brake system. The area of the piston in the master cylinder is 1.8 cm2, and that of the piston in the brake cylinder is 6.4 cm2. The coefficient of friction between shoe and wheel drum is 0.50. If the wheel has a radius of 34 cm, determine the frictional torque about the axle when a force of 44 N is exerted on the brake pedal.

    http://img690.imageshack.us/img690/2138/screenshot20121203at907.png [Broken]

    2. Relevant equations
    This I'm honestly not sure about. I initially thought that because pressure is distributed evenly throughout all parts of the fluid, it should be

    F1 / A1 = F2 / A2
    But as my solution shows, apparently this is not the case

    3. The attempt at a solution

    I did
    44 / 1.8 = F2 / 6.4
    F2 = 156.4 N
    Tfrictional = 156.4(0.5)(0.34) = 26.6 N *m

    However, all of the answer keys state to do
    44(1.8/6.4),
    which results in the answer 2.1 N*m
    Any help here? I'm really not sure what I'm doing wrong. THanks!
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Dec 4, 2012 #2

    haruspex

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    I agree with your result. (It would be crazy to have to push harder on the pedal than the resulting force on the brake shoe.)
     
  4. Dec 4, 2012 #3
    Hmm... the problem statement vs the picture.
     
    Last edited: Dec 4, 2012
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