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Torques: Rotational Equilibrium Mobile

  1. Oct 30, 2008 #1
    Torques: Rotational Equilibrium Mobile :)

    1. The problem statement, all variables and given/known data

    Please! I really need help figuring out how to make a balanced mobile!
    I would really appreciate anyones help!!!!

    Directions:
    Apply what you know about torques by making a mobile. Shown below are five horizontal arms with fixed 1- and 2-kg masses attached, and four hangers with ends that fit in the loops of the arms, lettered A through R. You are to figure where the loops should be attached so that when the whole system is suspended from the spring scale at the top, it will hang as a proper mobile, with its arms suspended horizontally. This is best done by working from the bottom upward. Choose the loops where the hangers should be attached. You may assume the horizontal struts and connecting hooks are practically massless compared to the 1- and 2-kg masses.

    __________________________________________________________________________________
    2. Relevant equations
    HERE IS A PICTURE! :)
    http://i22.photobucket.com/albums/b338/iloveyou2814/e19a2b53-385a-48ec-b0a2-440684444ae.gif

    _______________________________________________________________________________________________
    1. On the bottom level:
    You would connect point ____
    D E F G
    to point ____.
    A B C

    2. On the next to lowest level
    You would connect point ____
    H I K K
    to point ____.
    D E F G

    3. middle level
    You would connect point ____
    L M N O
    to point ____.
    H I K K

    4. next to top level
    You would connect point ____
    P Q R
    to point ____.
    L M N O

    5. top level
    The spring scale should be connected to point ____.
    P Q R

    6.Now that your mobile is complete, how many newtons would be indicated on the spring scale?


    3. The attempt at a solution
    I have tried for a really long time to make this mobile! But it just won't balance! I would show you it, but i don't have a digital camera anymore to take a picture of it because it is broken :(
     
    Last edited: Oct 30, 2008
  2. jcsd
  3. Oct 30, 2008 #2
    Re: Torques: Rotational Equilibrium Mobile :)

    How did you get on with hanging the bottom arm from the second-from-bottom arm?

    (Also, bear in mind that you don't need to hang each arm from its center.)
     
  4. Oct 30, 2008 #3
    Re: Torques: Rotational Equilibrium Mobile :)

    I am attaching each arm with hangers
    And i know that i can't hang each arm from its center because it won't balance that way,
    the "2"s and "1"s are weights, so i have to attach each arm to the next in different places. i just don't know where to attach them.
     
  5. Oct 30, 2008 #4
    Re: Torques: Rotational Equilibrium Mobile :)

    Is anyone going to help me!?
    i really could use come help :(
     
  6. Oct 31, 2008 #5
    Re: Torques: Rotational Equilibrium Mobile :)

    When I asked "how are you getting on with hanging the bottom arm from the second bottom arm?" I didn't realise you were actually trying to make the entire mobile. Unless you measure all the masses and distances exactly - and have massless rods (impossible in practice) it won't work out exactly in real life.

    I'll try rephrasing it - in which position should you provide the support for the bottom rod in order for it to balance. Think about people sitting on a seesaw if it helps.

    If you work out what the mobile should look like in theory, then you can build it practically after that (using slight alterations to make it balance in practice).
     
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