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A few questions from an already graded test on classical forces

  1. Nov 14, 2012 #1
    1. The problem statement, all variables and given/known data

    A mass m is suspended from a spinning rod by two massless cords of equal length L, held fixed at points along the rod D meters apart. It is observed that m moves in a circle with a tangential velocity v, find the ratio of the tensions between the two supporting cords

    2. Relevant equations

    Newton's three laws of motion

    3. The attempt at a solution

    I didn't get terribly far on this one; I focused too much time on frictional forces during study.

    I set up the problem, showing an isoceles triangle pointing rightward (with the mass m at the nexus), and angles θ1 and θ2. I didn't get too much farther due to time constraints while actually taking the test, and the instructor himself won't be available to answer it for close to a week. Any help is appreciated.

    I can post an image of what I did once I get home, in case it helps.
     
    Last edited: Nov 14, 2012
  2. jcsd
  3. Nov 14, 2012 #2

    SammyS

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    Do you have a figure you can post or can you describe the situation in more detail?
     
  4. Nov 14, 2012 #3

    haruspex

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    I assume the rod is vertical. It would have been nicer of the questioner to make that clear, particularly since 'suspended' makes it sound otherwise.
    Why did you put two different angles?
    Make up some variable names for the two tensions and see what equations you can write down for the vertical and horizontal forces and accelerations.
     
  5. Nov 14, 2012 #4
    Ok, here's the image with picture.

    I wrote down two angles, thinking that I needed to find a ratio of the angles first. Seems to be rather poor reasoning in hindsight.
     

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  6. Nov 15, 2012 #5

    SammyS

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    For an isosceles triangle, θ1 = θ2. Right?

    attachment.php?attachmentid=52971&d=1352956574.jpg
     
  7. Nov 15, 2012 #6
    In that case, because θ1 = θ2 and L1 = L2, would the tensions be equal, and thus in a 1:1 ratio?
     
  8. Nov 15, 2012 #7

    SammyS

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    No. the tensions would not be equal.

    Draw a free body diagram for the mass, m.
     
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