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Need help with my Swinging Ball problem please

  1. Oct 5, 2011 #1
    1. The problem statement, all variables and given/known data

    A mass m = 16.0 kg is attached to the lower end of a massless string of length L = 63.0 cm. The upper end of the string is held fixed. Suppose that the mass moves in a circle at constant speed, and that the string makes an angle theta = 17o with the vertical, as shown in the figure. How long does it take the mass to make one complete revolution?

    2. Relevant equations

    T = 2 pi r / v

    F = m a

    Ac = v^2 / r

    3. The attempt at a solution

    m = 16
    L = 0.62 m
    θ = 17

    Drew a FBD...
    It shows mg acting down upon the object, and T diagonally up.


    Fx = m * ac
    T sin theta = m ac
    T sin theta = m * v^2/r
    T sin theta = m * v^2/ L sin theta

    Fy = m * ac
    T cos theta - mg = 0
    T cos theta = mg
    T = mg /cos theta

    Use the above and sub it in..

    (mg/cos theta) sin theta = m (v^2) / L sin theta

    √ ((( L (sin(theta))^2) g sin(theta))) / cos theta ) = V

    V = (sqrt( (.63 (sin^2(17)) (9.8) sin(17) )/ cos(17)))

    Use formula... T = 2pi r / v

    (2pi(0.63*sin(17)))/(sqrt( (.63 (sin^2(17)) (9.8) sin(17) )/ cos(17)))

    I get.. 2.88 seconds.

    Where did I go wrong?? Thanks for your time!
     

    Attached Files:

  2. jcsd
  3. Oct 5, 2011 #2
    Is there anything else I need to include to get help?
     
  4. Oct 5, 2011 #3

    SammyS

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    Patience.

    Draw a Free Body Diagram for the ball. (never mind.)
     
  5. Oct 5, 2011 #4
    **** you all, this website is ********. you guys never actually respond and when you do its useless. suck a cows dick. I'm out.
     
  6. Oct 5, 2011 #5
    I did it perfectly..... all I did wrong was that I unnecessarily squared an sin theta and you faggots couldn't tell me that. this site is useless...
     
  7. Oct 5, 2011 #6

    SammyS

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    I get that [itex]\displaystyle v^2=\frac{gL\,\sin^2(\theta)}{\cos(\theta)}\,.[/itex]

    It's hard to say what you have for v. There is a severe case of unbalanced parentheses.
     
  8. Oct 5, 2011 #7

    SammyS

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    That's probably NOT the way to get help.
     
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