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Oscillations - mass suspended from string

  1. Jun 21, 2014 #1
    1. The problem statement, all variables and given/known data
    the answer is a for this question. but i dont understand how to get the solution.


    2. Relevant equations



    3. The attempt at a solution
    mg-k(l-a+b)=ma

    since mg=k(l-a)

    -kb=ma

    a=-(k/m)b

    k/m=a/b

    so my ans is sqrt root( a/b)
     

    Attached Files:

  2. jcsd
  3. Jun 21, 2014 #2

    NascentOxygen

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    Hi, desmond. Welcome to the forum.

    a, b, and l are constants, so the equation you came up with shows a fixed, constant acceleration. As you realize, this isn't correct; a fixed acceleration is not a characteristic of oscillatory motion. Somewhere in that equation you'd need a variable x which is the extension at any moment.

    It might be easier if you were to start with the options, and maybe eliminate them one by one? Look for a reason why any particular option could not be the formula for ω2.
     
    Last edited: Jun 21, 2014
  4. Jun 22, 2014 #3
    so if the option of (a/b) is given, can i choose it?
     
  5. Jun 22, 2014 #4

    Orodruin

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    Try getting to the differential equation given in the problem by considering what forces are acting on the mass for an arbitrary displacement.
     
  6. Jun 22, 2014 #5
    can you explain further?
     
  7. Jun 22, 2014 #6

    NascentOxygen

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    Do you understand the mathematics behind x with a pair of dots above it, as it appears in the textbook question in your attachment? (Sometimes it is written x’’ )
     
    Last edited: Jun 22, 2014
  8. Jun 22, 2014 #7

    NascentOxygen

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    Choose it if you wish. It's wrong though.

    Oh, I just noticed: aren't you using the letter "a" for two different things in your equations in this thread??
     
    Last edited: Jun 22, 2014
  9. Jun 22, 2014 #8

    rude man

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    Hint: determine the spring constant k, then use the formula for radian frequency w of a mass bobbing up & down:
    w = w(k,m).

    If you don't know the formula you'll have to proceed per post #4.
     
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