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Object on a string

  1. Oct 9, 2007 #1
    1. The problem statement, all variables and given/known data

    The string in the Figure is L = 109.0 cm long and the distance d to the fixed peg P is 74.1 cm. When the ball is released from rest in the position shown, it will swing along the dashed arc. How fast will it be going when it reaches the lowest point in its swing?How fast will it be going when it reaches the highest point in its swing?

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    I am totally lost as to how to even start this problem. I drew a free body diagram, but I do not know what equations I need to use. Could someone please walk me through this problem?
     
  2. jcsd
  3. Oct 9, 2007 #2

    Dick

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    You want to use energy conservation equations. Do you know any of those that might apply?
     
  4. Oct 9, 2007 #3
    I have no idea which ones would apply.
     
  5. Oct 9, 2007 #4

    Dick

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    You aren't trying. KE+PE=constant. KE=(1/2)*m*v^2. PE=mgh. Take it from there.
     
  6. Oct 9, 2007 #5
    I've never taken physics before, so this is my first time ever doing this stuff. So are these the only equations that apply?
     
  7. Oct 9, 2007 #6

    Dick

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    They aren't the only ones that apply. But they are the only ones that you need.
     
  8. Oct 9, 2007 #7
    I don't understand how you take the length of the string and incorporate that into those equations to get a speed. Could you please explain that
     
  9. Oct 9, 2007 #8

    Dick

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    Do you know what the equations that I wrote mean? Have you seen them before? Do you know conservation of energy?
     
  10. Oct 9, 2007 #9
    I understand what those equations mean, I just don't know how to apply them to this problem. I have seen these equations before.
     
  11. Oct 9, 2007 #10

    Dick

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    Ok, then put h=0 to be the bottom of the arc. What the total energy of the ball at it's initial position?
     
  12. Oct 9, 2007 #11
    At it's initial position it would just be the PE=mgh, but what is the mass?
     
  13. Oct 9, 2007 #12

    Dick

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    m is m. It will cancel if you are patient. What's h in terms of your diagram?
     
  14. Oct 9, 2007 #13
    h would be the length L of the string. 109 cm
     
  15. Oct 9, 2007 #14

    Dick

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    Ok. So total energy is mgL. Now what are KE and PE at the bottom of the arc? Answer both to earn points. Remember their total is the same energy that you started with, mgL.
     
  16. Oct 9, 2007 #15
    KE would be mgL, and PE would be 0. Is that right?
     
  17. Oct 9, 2007 #16

    Dick

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    Right, but KE is also (1/2)mv^2. What's v at the bottom of the arc? Get that and you have half of the points.
     
    Last edited: Oct 9, 2007
  18. Oct 9, 2007 #17
    I am not sure what the velocity would be.
     
  19. Oct 9, 2007 #18

    Dick

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    KE=(1/2)mv^2. KE=mgL. I think we've agreed on that. Can you solve for v in terms of L?
     
  20. Oct 9, 2007 #19
    so v would be the square root of g*L/.5?
     
  21. Oct 9, 2007 #20

    Dick

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    Yessssssss. Now what's PE at the top of the circle of radius r. What's KE at the top of the circle? The total is still mgL. Energy is conserved. I'm hoping you'll get this soon. It's bedtime, help me out.
     
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