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Moment of Inertia Ball and Stick Collision

  1. Jan 2, 2009 #1
    1. The problem statement, all variables and given/known data
    A mass is dropped from height (h) onto one end of a stick of mass (m) and of length (l) pivoted around the opposite end. The moment of inertia of the stick is 1/3ML^2. Upon collision the mass adheres to the stick.

    a. Find speed of mass just before impact
    b. Find angular speed of the system immediately after impact
    c. Find linear speed of the mass (m) at its lowest point (when stick is vertical)
    d. Determine the mechanical energy lost as a result of the collision.

    2. Relevant equations
    Conservation of angular momentum
    angular kinematics equations

    3. The attempt at a solution

    For the first part I think I found easily with vf^2=Vo^2+2ah which will get you vf=(2ah)^1/2
    The last three parts I am very confused about. I have that Io wo=If wf The last part I would think I would use the kinetic energy at the start and at the bottom.
  2. jcsd
  3. Jan 3, 2009 #2


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    Gold Member

    Remember the relationship between [tex]\omega[/tex] and velocity [tex] v = r \omega[/tex] also remember in this incident the complete momentum equation entails linear momentum so

    [tex]mv_{i} + I \omega_{i} = mv_{f} + I \omega_{f}[/tex]

    remember there is no initial rotational momentum

    I am a tad rusty right now since its been over 2 months since I've last seen this stuff (vacation lol)

    I know for sure this model holds for conservation of energy though

    comment on and/or let me know how it works out

    Edit: remember your [tex]v_{f}[/tex] (from kinematics) = [tex]v_{i}[/tex] (in the momentum equation)
  4. Jan 3, 2009 #3
    Does Mo*Vo= I*Wfinal and then is I=(1/3)(2M)(L^2) this will cause the angular speed to be really complicated looking (3/2L^2)*(2gh)^(1/2) and is no mechanical energy lost in the system?
  5. Jan 5, 2009 #4
    Also, I am having trouble finding the equation for linear motion compared to rotational, does anyone have a link to the trig equation?
  6. Jan 5, 2009 #5
    v = rw
    linear velocity = radius x angular speed
  7. Jan 5, 2009 #6

    Doc Al

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    Staff: Mentor

    What's the angular momentum of the system before the collision?
    What's the rotational inertia of the system once the ball attaches to the stick?
    Use this to find the rotational speed just after the collision.

    What's the energy of the system just before the collision? Just after the collision?
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