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Homework Help: Confusing Angular Momentum problem with Door

  1. Nov 21, 2009 #1
    1. The problem statement, all variables and given/known data

    A solid wood door of width 1.00 m and height 2.00 m is hinged along one side and has a total mass of 49.0 kg. Initially open and at rest, the door is struck at its center by a handful of sticky mud with mass 0.500 kg, traveling perpendicular to the door at 14.0 m/s just before impact. Find the final angular speed of the door with the mud stuck to it.

    I'm totally lost on this one. Four different approaches, four very different answers, all of them wrong.

    Please help.
  2. jcsd
  3. Nov 21, 2009 #2


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    Homework Helper

    As the door is hinged, there is a force acting from the hinge so the momentum will not conserve during the impact. Neither the energy will conserve as there is an "inelastic" collision between mud and door. There is one quantity that conserves: the total angular momentum of the system door+ mud ball. Calculating the angular momentum both the piece of mud and the door with respect to the hinge, their sum is the same before and after the impact.

  4. Nov 21, 2009 #3
    I believe I tried that, but I must be doing something wrong with that angular momentum calculation.

    How do you calculate the angular momentum of the mud before it hits the door? Is it (mass of mud * velocity of mud * distance from the hinge)? Since it hits at the middle, that distance would then be 0.5 meters. And then when calculating the moment of inertia of the system that's (1/3)MR^2 for the door and (MR^2) for the mud as a point mass.

    Yet something must be incorrect because using these equations isn't generating the correct answer.
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