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Quick energy question

  1. Apr 9, 2005 #1
    a wreacking ball is used to demolish a building. The 600-kg ball starts from rest, with it's 37-m-long cable making a 22 degrees angle with the vertical. it strikes the building when the cable is vertical. What is the speed of the ball on impact?

    it doesnt seem like a hard question, but i cant figure it out.

    [tex]U_o =mgL(1-cos(\theta))[/tex]
    [tex] K_0 = 0[/tex]

    [tex]U_f = 0[/tex]
    [tex]K_f = 1/2mv^2[/tex]

    [tex]mgL(1-cos(\theta)) = 1/2mv^2[/tex]
    [tex]V= \sqrt{\frac{gL(1-cos(\theta))}{2}}[/tex]

    plugging in known values, i get 3.6 but the book gets 7.3 m/s. am i missing something?
  2. jcsd
  3. Apr 9, 2005 #2


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    Science Advisor

    Your final equation should be:

    [tex] \blacktriangleright \blacktriangleright \ \ \ \ \color{red} v \ = \ \sqrt{2gL(1 - \cos(\theta))} [/tex]

    Use this correct version, and you'll obtain the book answer.

    Last edited: Apr 9, 2005
  4. Apr 9, 2005 #3
    oops your right, thanks for the help
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