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Homework Help: Lever rotating around an axis.

  1. May 24, 2007 #1
    1. The problem statement, all variables and given/known data

    A lever of length 3L can rotate freely around a horizontal axis O.
    The end of each arm "2L" and "L" of the lever has one mass : "4m" and "9m" , respectively.

    Find the velocity of mass "9m" at the bottom.
    Consider that the lever was initially at rest in the horizontal position.

    2. Relevant equations
    The problem is in a chapter of work and energy, before the conceps of torque and moment of inercia, so i guess that only energy and work can solve this.
    But any solution/hint with any method will be welcome.

    3. The attempt at a solution
    I'm trying to apply conservation of energy, but i'm confused if the whole system is conservating energy or only part of it.

    I really need a hint of how start this problem or if possible a solution of it.

    Thanks in advance!

    Attached Files:

  2. jcsd
  3. May 25, 2007 #2


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    Homework Helper

    What forces are acting on the system?
  4. May 25, 2007 #3
    I believe that only the gravitational forces are acting on the system "lever+masses". I can't see any other possible force able to do work.
  5. May 25, 2007 #4

    Doc Al

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    Staff: Mentor

    Since everything is connected, treat the system as a whole. How does the mechanical energy change as the system rotates?
  6. May 25, 2007 #5
    The mechanical energy of the system as whole is doesn't changing ... right?
    (Does the support O of the lever is doing work?) .
  7. May 25, 2007 #6

    Doc Al

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    Staff: Mentor

    I interpret "rotate freely" as saying that there is no friction, and thus no mechanical energy lost to friction. The support O does no work, since it doesn't move.
  8. May 25, 2007 #7
    Thank you very much for the help DocAl. The sentence quoted above is exactly what i was looking for.
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