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Homework Help: Fluids - Pascal's Principle / Negative vs Positive

  1. Dec 9, 2009 #1
    1. The problem statement, all variables and given/known data

    The handle of a hydraulic jack is 15 cm long and is pivoted 2.5 cm from the input piston which has a radius of 0.60 cm. The output piston has a radius of 1.2 cm. What weight could be lifted by the jack if the person pushing on the handle is to exert no more than 110 N of force?

    http://edugen.wiley.com/edugen/courses/crs2216/cutnell3550/ssg/cutnell3255c11/image_n/nw0002-y.jpg


    3. The attempt at a solution

    Silly question...I doubt I'm right, as I'm hardly an authority to go up against a text book. But that being said...

    I'm confused as to why the textbook (Cutnell & Johnson Physics Study Guide 7th ed.) says that F is positive when F1 is negative? They're both pointing down...so...aren't they both negative??? F2 is positive and pointing upwards, that makes sense. But why F is also positive completely eludes me...

    Am I missing something completely obvious?

    On a side note - I hope I'm not violating any copyright laws by posting that diagram. I just want the answer, not a prison term!
     
  2. jcsd
  3. Dec 10, 2009 #2
    Any thoughts?
     
  4. Dec 10, 2009 #3

    stewartcs

    User Avatar
    Science Advisor

    Perhaps they are summing the moments about the hinge in order to determine the force F1. So maybe they chose the + moment in the CCW direction (making F x d positive). Then the reaction at F1 (F1 x (d-2.5)) would be negative since it would cause a CW rotation.

    Just a thought.

    CS
     
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