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Homework Help: Work and Energy of wrecking ball

  1. Feb 16, 2015 #1
    1. The problem statement, all variables and given/known data
    A wrecking ball swings at the end of a 10.0-m cable on a
    vertical circular arc. The crane operator manages to give the ball a
    speed of 6.00 m/s as the ball passes through the lowest point of its
    swing and then gives the ball no further assistance. Friction and
    air resistance are negligible. What speed v does the ball have
    when the cable makes an angle of 30.0° with respect to the
    vertical?

    2. Relevant equations
    Gravitational potential energy

    3. The attempt at a solution
    image.jpg

    The answer must be 3.1 m/s
     
  2. jcsd
  3. Feb 16, 2015 #2

    TSny

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    2017 Award

    When calculating r + k, note that the base of the triangle is not 5 m.

    You might try a different construction. Draw a horizontal auxiliary line from the final position of the ball over to the vertical string at the initial position.
     
  4. Feb 16, 2015 #3
    Sorry, I saw my mistake xD .

    If I draw a line from initial to final position, I get an isosceles triangle. Still I don't know how to find the hight.
     
  5. Feb 16, 2015 #4

    TSny

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    2017 Award

    Here's the horizontal line I was suggesting.
     

    Attached Files:

  6. Feb 16, 2015 #5
    Solved it, thanks!
     
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