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Angular speed help please

  1. Oct 8, 2009 #1
    1. The problem statement, all variables and given/known data

    A thin rod of length 1,5m is oriented vertically, with its bottom end attached to the floor by means of a frictionless hinge. The mass of the rod may be ignored compared to the mass of an object fixed to the top of the rod. The rod, starting from rest, tips over and rotates downward.

    a) what is the angular speed of the rod just before it strikes the floor?

    b) What is the magnitude of the angular acceleration of the rod just before it strikes the floor?

    2. Relevant equations

    [tex]\omega[/tex] = [tex]\theta[/tex]/t

    [tex]\theta[/tex] = s/r

    3. The attempt at a solution

    if the rod rotates from vertical to flat on the floor then the angular displacement must be 90/57,3 right? 1,57 rad
    not sure how to get the time interval though

    please help
  2. jcsd
  3. Oct 8, 2009 #2


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    Homework Helper

    At the top of the rod what is the potential energy and the kinetic energyof the object?
    Just before the object strikes the floor, what is its potential energy and the kinetic energy?
    From these hints can you find the final velocity of the object?
  4. Oct 8, 2009 #3
    k at the top PE is mgh... dunno the value of m though.

    KE is 0

    at the bottom PE is 0

    and KE is 1/2 mv2

    but i dont know the value of m...
  5. Oct 8, 2009 #4


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    Homework Helper

    According to the conservation of energy, total energy at the top = total energy near the floor. Equate them. You will get the value of v.
  6. Oct 8, 2009 #5
    so simple :D dammit how do u know when to use these principles of conversation? its so confusing :(

    so by doing that i got v= -29,4 but i need angular speed.

    so VT = [tex]\omega[/tex] r ?

    then i get -19,6 rad/s
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