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Homework Help: Question on S.H.M

  1. Mar 4, 2008 #1
    1. The problem statement, all variables and given/known data

    provided in picture

    2. Relevant equations
    provide


    3. The attempt at a solution
    I know how to show ,but I don't understand why we consider one mass only

    [PLAIN][PLAIN]http://b.imagehost.org/0195/questionb.jpg [Broken]
     
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Mar 4, 2008 #2

    Doc Al

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    Staff: Mentor

    That looks like two masses connected by a spring. You describe the motion of each mass (with respect to the center) separately, but they are definitely connected.
     
  4. Mar 4, 2008 #3
    they strecthed at the same time.
    so

    ma =-kx
    a=-(k/m) x
    ma=-kx
    a= -(k/m)x

    but I don't know how to combine as

    a=-(2k/m)x

    I think it should be a=-(2k/2m)x = -(k/m)x
     
  5. Mar 4, 2008 #4
    I believe that instead of stretching both, you could hold one down and stretch the other twice as far.
     
  6. Mar 4, 2008 #5
    That means

    ma = -k(2x)?
    a=(-2k/m)x ?

    I am doubt why ma is used instead of 2ma ?
    the whole mass of system is 2m ?
     
  7. Mar 5, 2008 #6

    Doc Al

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    Staff: Mentor

    If x is the displacement of each mass, the amount of stretch in the spring is 2x.
     
  8. Mar 5, 2008 #7
    but why should we use

    ma=-2kx
    but not
    2ma=-2kx ?
    since there are two bob with mass m..
     
  9. Mar 5, 2008 #8

    Doc Al

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    Staff: Mentor

    Because we are analyzing the motion of each mass separately:

    What's the mass? m
    What's the stretch of the spring? 2x
    What's the force on each mass? -2kx
     
  10. Mar 5, 2008 #9
    Thanks Doc Al.
    you do help me a lot :devil:
     
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