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Homework Help: Please help this problem!~

  1. Oct 17, 2007 #1
    Please help this urgent problem!~

    1. The problem statement, all variables and given/known data

    A ball is tied to a light string L=1.39m . What is the minimum horizontal speed of the ball which will ensutre that the ball makes a full vertical circle?

    2. Relevant equations

    I still struggle how this answer is 8.25

    3. The attempt at a solution

    8.25 <-- this is correct answer from professor but I don't know how it became
  2. jcsd
  3. Oct 17, 2007 #2
    8.25 m/s?

    well what do you mean by full 'vertical' circle? a full circle? meaning one whole rotation right?

    Well L=1.39m should equal the radius(r) of this circle right?
    So i see that 2*pi*r is the circumference and effectively the distance the ball has to cover. which i get to be 8.734m.

    I don't know what do to from here.
  4. Oct 17, 2007 #3
    I think it is full vertical circle..I still trying another way..Please keep on eye this~
  5. Oct 17, 2007 #4
    Also L= should equal to r~
  6. Oct 17, 2007 #5


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    Homework Helper

    Think centripetal motion... what does the velocity at the top of the loop need to be?

    then think conservation of energy... energy at the bottom must equal energy at the top.
  7. Oct 17, 2007 #6
    I put the mgr=1/2 * m * v**2 but I can't find.. any ideay someone has this?
  8. Oct 17, 2007 #7
    Draw yourself a picture. The energy at the top isn't mgr.
  9. Oct 17, 2007 #8


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    Homework Helper

    First use centripetal acceleration to find what the velocity at the top of the loop needs to be...

    Then use conservation of energy. How high above the bottom of the loop is the top of the loop? you also need to consider the kinetic energy at the top of the loop.
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