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Homework Help: Dentist Chair

  1. Nov 26, 2007 #1
    1. The problem statement, all variables and given/known data
    A dentist's chair of mass 207kg is supported by a hydraulic lift having a large piston of cross-sectional area 1403.0cm^2. The dentist has a foot pedal attached to a small piston of cross-sectional area 78.0cm^2. What force must be applied to the small piston to raise the chair.

    2. Relevant equations
    Fp/Ap = Fc/Ac

    3. The attempt at a solution
    Fp = Fc/Ac * Ap
    Fp = 207kg/1403 * 78 = 112.78

    Okay so I have already gotten the answer 112.78N but I don't understand a few things about this question...
    - Why does Fp/Ap = Fc/Ac??
    - Why is 207kg relate to 1403cm^2?
    - What is the subscripts p and c???
  2. jcsd
  3. Nov 26, 2007 #2


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    Homework Helper

    Suggestion in your calculations when you said...
    F=mg...so the force should be the weight..so you have to multiply by 9.81Nkg^-1 or whatever value you take as g and convert the cm^2 to m^2

    "Why does Fp/Ap = Fc/Ac??"

    Does this not mean that [itex]P_p=P_c[/itex] which just basically says that pressure is transmitted equally throughout a fluid...i.e. the pressure at one end is equal to the pressure at the other.

    the subscripts p and c would be piston and chair respectively
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