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How longs a peice of string

  1. Mar 22, 2007 #1
    1. The problem statement, all variables and given/known data
    In this case how tights a peice of wire with a ball, at one end and a pivot P at the other. A wire from the pivot P suspends a ball, m.

    Problem:Find T(r) for the values of m, v and g (in terms of r).



    2. Relevant equations

    The tension or force in the wire, T, when the ball m is swinging through the lowest point is given by the function:

    T(r)=mg+(mv^2/r) where m=1.0 kg, v=7.75 ms–1, g = 9.81 ms-2 T is in Newtons

    r is the disrance from the pivot P to the centre of the ball when the wire is stretched.When the wire is not stretched, its length is 2.89m.




    3. The attempt at a solution

    0=mg+(mv^2/r)
    -mg=mv^2/r)
    -mg(r)=mv^2
    r=-mv^2/mg therefore r=-1x7.75^2/1x9.81 =-6.123m 3dp

    Just wondering if Im on the right track here I dont see how r could be a negative number and if not how can it be twice the length when stretched?
     
    Last edited: Mar 22, 2007
  2. jcsd
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