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How to solve this pendulum problem?

  1. Jun 8, 2017 #1
    1. The problem statement, all variables and given/known data

    For number 3,4,5
    IMG_1092.png

    IMG_1093.png

    https://s4.postimg.org/qbp3xzq65/IMG_1092.png

    https://s22.postimg.org/u220j60sx/IMG_1093.png


    2. Relevant equations
    Centripetal force = m v^2/R
    T = centripetal force + w
    Sigma F = m.a

    3. The attempt at a solution
    Number 3. Find the v at D point using mechanic energy eternity
    While kinetic energy at A point is zero, and the potential energy is max (height max)
    So, 1/2mvD^2 + mg.hD = 1/2mvA^2 + mg.hA
    i dont know the height at D point....

    After i find the v at D
    I can find T
    Which T = centripetal Force + weight
    T = mvD^2/R + m.g

    Number 4 almost the same
    The string doesnt bend (what exactly it means...?)
    So, centripetal force > weight??
    m.vD^2/R > m.g ...(1)
    But why the centripetal at point D has opposite direction than weight force? Is it cuz the string stuck at the nail at C point??

    Find the v at D using mechanical energy eternity then add v to ...(1)

    Number 5 i still dont know...
     

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    Last edited: Jun 8, 2017
  2. jcsd
  3. Jun 8, 2017 #2

    BvU

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    Hello Helly, :welcome:

    Unfortunately your pictures don't show up in your post. I get:

    upload_2017-6-8_13-33-27.png

    Can you try again ?
     
  4. Jun 8, 2017 #3
    ok.. thanks for report. I've changed the pic.. :)
     
  5. Jun 8, 2017 #4

    BvU

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    with ##v_A = 0 ##.
    make a sketch of the situation and you know it. Note that CB = CD.
     
  6. Jun 8, 2017 #5

    haruspex

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    You know the length of the string, and you know how far C is below A. How far above C is D?
     
  7. Jun 8, 2017 #6
    is the height of D start from C or B ........???

    CD = a-b
    BC = a-b
    if D start from C then height of D = a-b
    if D start from B then height of D = 2(a-b)
     
  8. Jun 8, 2017 #7

    BvU

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    Check your energy conservation equation: If A is at height zero, you must take height of D wrt the same referernce.
     
  9. Jun 8, 2017 #8
    if A = height zero.. then kinetic energy is max....
    then B point the kinetic energy is zero... how can it be...
     
  10. Jun 8, 2017 #9

    BvU

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    It was you who chose height A = 0 ! Therefore at point B the height is -a and the energy conservation still reads $$
    {1\over 2} mv_B^2 + m g h_B = {1\over 2}mv_A^2 + mg\,h_A \quad {\rm or} \\
    {1\over 2} mv_B^2 - mga = 0 $$
     
  11. Jun 8, 2017 #10
    While kinetic energy at A point is zero, and the potential energy is max (height max)
    hmm I see..
    then it said the string doesn't bend, how that means Sir??
     
  12. Jun 8, 2017 #11

    BvU

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    As long as you have tension in a string, it does not bend but remains taut.
    First work out number 3, once you have that, then number 4 is easy

    By the way, what did you get for number 2 ?

    Do you realize image 1093 is extremely fuzzy ?
     
  13. Jun 8, 2017 #12
    number 2? I get sqrt 2ga. do you even know the questions Sir.. but I'm still trying for number 3...
     
  14. Jun 8, 2017 #13

    BvU

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    Very unsharp, in image 1093 under (2) It says: Find the magnitude of the tension in the string just before the small ball reaches B.
    ##\sqrt{2ga}\ ## is not among the choices. But also, 2mg ...
    Is the wrong answer (as I already suspected :rolleyes: ) .

    You need to get this right: the same issue also needed in (3).
     
  15. Jun 8, 2017 #14
    lol.. sorry Sir. I meant 3mg..
     
  16. Jun 8, 2017 #15

    BvU

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    Agree with 3mg.
     
  17. Jun 8, 2017 #16
    oh.... it's my mistake... lol i'll try again number 3.. how do you even know the question for number 2, from the pic there's no number 2
     
  18. Jun 8, 2017 #17

    BvU

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  19. Jun 8, 2017 #18
  20. Jun 8, 2017 #19

    BvU

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    I know that.
     
  21. Jun 8, 2017 #20
    I don't even understand... How to solve number 3....
     
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