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Homework Help: What Is The Complete Formula Of A Lever?

  1. Sep 25, 2009 #1
    In trying to solve a problem, I was using the simple F1D1 = F2D2 as the balance point of a lever. My limited physics knowledge knew no better.

    My hypothetical involved using the "short side" of a lever to lift a weight on the long side of a lever.

    After thinking about it... I realized I couldn't have a big cast iron lever and expect to balance it by placing 4 quarters on one end. I realized that the weight of the lever has to be factored in.

    Since so many web pages talk about levers but never talk about this aspect, I am here hoping someone can tell me (or link me to) the COMPLETE formula of a lever.

    Thank you for any help.

    PS. I apologize if this is a redundant question, or if this is the wrong forum for this question. This place is so big that it's hard to find things.
  2. jcsd
  3. Sep 25, 2009 #2

    Doc Al

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    Staff: Mentor

    To include the weight of the lever itself, just realize that its weight acts at its center of mass (for the purposes of calculating the torque it would produce).
  4. Sep 25, 2009 #3
    My original formula, that did not include the weight of the lever, was...

    F1 x 2 feet > F2 x 8 feet

    As you can see, the lever is 10 feet long, and I want to lift the long side of the lever using the short side of the lever (hence, the ">" sign).

    The center of mass of the entire lever would be at the 5 foot mark (3 feet to the right of the fulcrum). Let's say the entire lever is uniform and weighs 100 pounds.

    Is my formula now F1 x 2 feet > (F2 x 8 feet) + (3 feet x 100 lbs)

    Is that correct?
  5. Sep 25, 2009 #4

    Doc Al

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    Staff: Mentor

    Looks good to me.
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