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Homework Help: Tension and centripital motion help!

  1. Sep 22, 2005 #1
    Two identical blocks are tied together with a string and placed along the same radius of a turntable that is spinning about its center. The inner block is 3 cm from the center and the outer block is 6 cm from the center. The coefficient of static friction between the turntable and the blocks is ┬Ás = 0.74, and the string is taut.

    I have determined that w = 12.7 rad/s

    Now suppose that the blocks each have a mass m = 35 g. For the value of w you just found, what is the tension in the string?

    For some reason, I cannot seem to get the right answer. SOMEONE PLEASE HELP!!!!
     
  2. jcsd
  3. Sep 23, 2005 #2

    Doc Al

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    Staff: Mentor

    Show your work and how you solved for the tension and we can take a look.
    The same equations used to solve for [itex]\omega[/itex] (the maximum rotational speed without slipping) will include the tension.
     
  4. Sep 23, 2005 #3
    my equations i used were:
    1) T - umg = m R1 w^2
    2) -T - umg = m R2 w^2

    i was able to get w, but then i plug values back in and its not accepting my answer for T as being correct. i have no idea what im doing wrong. please help!
     
  5. Sep 23, 2005 #4

    Doc Al

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    Staff: Mentor

    For one thing, your signs are messed up in those equations. Choose a sign convention: for example, make towards the center positive, away from the center negative. Rewrite those equations accordingly. (The way they are written now, [itex]\omega^2[/itex] is negative!)
     
  6. Sep 23, 2005 #5
    ok i realized i typed in the wrong ones...i had those at first, the new ones are
    1) -T + umg = m r1 w^2
    2) T + umg = m r2 w^2

    i believe that is how i got 12.7 for w (i have so much work here and half of it is wrong, im not sure which is which anymore). I just tried solving for T though and it is still not right. ah im so confused now!
     
  7. Sep 23, 2005 #6
    Ok maybe those equations aren't right either...I can't seem to get the 12.7 for w again, tho I know that is right. I have no idea what I'm doing anymore!
     
  8. Sep 23, 2005 #7

    Doc Al

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    Those equations are correct. Show how you solved for T and what you got.

    (To find [itex]\omega[/itex], start by adding those two equations.)
     
  9. Sep 23, 2005 #8
    to find T i had:

    T = umg - m r1 w^2
    as well as
    T = m r2 w^2 -umg

    m=.035kg
    r1=.03m
    r2=.06m
    w=12.7 rad/sec
    u=.74
    g=9.81

    i plug those in and get
    T=.0847

    which apparently is right...i had that before i dont know y it wouldnt take it as being correct...thanks for all you help!!!!
     
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