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Homework Help: Centre of mass of this boomerang?

  1. Apr 3, 2016 #1
    1. The problem statement, all variables and given/known data
    I have an image with the question here: http://imgur.com/TGG9VTx.jpg
    The red text is the answer. I havent been able to get it without contradicting what Ive been told.

    2. Relevant equations
    I know m1x1 + m2x / m1+m2 is centre of mass in 2d plane, but I dont know how that applies here. Also vectorR=vectorR1x1 +vectorR2m2 / m1 + m2 is for the 3d, but I dont know how to apply that as well. Im dumb

    3. The attempt at a solution
    tried using (20i + 2j + .25k) (dividing all components by 2) then multiplying by 150 (mass of block) twice then dividing by 300. didnt get the answer.

    Then tried 150(20)+150(20) / 300, got 20 as answer. Wont get 10 unless I make x = 10, which I dont why I would do that.

    ANY help appreciated. Thanks
  2. jcsd
  3. Apr 3, 2016 #2


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    Science Advisor

    Since a boomerang is symmetric, why not take advantage of that?
    Then, where is the center of mass of a stick with uniform cross section?
    That should be enough hints to find the answer. :wink:
  4. Apr 3, 2016 #3
    Thank you very much for the help, but im afraid im not quite sure i understand? :sorry:
  5. Apr 3, 2016 #4


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    Gold Member

    That will give you the mass centre of one stick in a particular coordinate system. But that coordinate system is tailored to that stick. You should get a very different answer for the mass centre of the other stick in that same coordinate system. You are not going to get a sensible result by adding up vectors from different coordinate systems, one for each stick.
    Pick a coordinate system that makes sense for both sticks.
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