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How fast is this bead traveling down the wire

  1. Dec 14, 2009 #1
    A big metal bead slides due to gravity along an upright frictiion-free wire. It starts from rest at the top of the wire as shown in the sketch. How fast is it traveling as it passes.

    http://https://www.physicsforums.com/attachment.php?attachmentid=22509&stc=1&d=1260854052 [Broken]

    Point B?
    Point D?
    Point E?
    At what point does it have the maximum speed?
    I got B for this because it goes down really fast.
    1. The problem statement, all variables and given/known data



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    3. The attempt at a solution
     

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    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 15, 2009 #2

    AEM

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    Re: Energy!

    The way to do this question is to use conservation of energy.

    Gravitational PE at top goes into PE + KE as the bead slides down.
     
  4. Dec 15, 2009 #3
    Re: Energy!

    so what do we exactly do?
    because we dont have the number to d and e. i think i get b.
     
  5. Dec 15, 2009 #4

    AEM

    User Avatar

    Re: Energy!

    I think you might want to review the concept of conservation of energy for objects falling in a gravitational field. Briefly, you pick a reference level where potential energy = 0. When the object is above that level its PE is given by mgx where x is its current height. The conservation of energy equation goes as

    Energy at the start = [itex] mgx + \frac{1}{2} m v^2 [/itex]

    Here your energy at the start is mgh. Now look at your drawing carefully. Where IS the object going the fastest? This is subtle because it's where the PE is the LEAST. What is the relationship between points b, d, e? Which one is highest? Or they all at the same height?
     
  6. Dec 15, 2009 #5
    Re: Energy!

    hmm i think i get it so the answer will be some kind of formula then right??
     
  7. Dec 15, 2009 #6

    AEM

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    Re: Energy!

    Actually you can get a numerical answer for the speed at points b, d, e if you pick your reference level (PE = 0, x = 0) along the horizontal line passing through b, d, e. You cannot get a numerical answer for point c because you don't know how far below point b it is.
     
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