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Homework Help: Spring and block of mass problem

  1. Oct 7, 2009 #1
    1. The problem statement, all variables and given/known data

    a block of mass m lies on a horizontal frictionless surface and is attached to one end of a horizontal spring (spring constant k) whose other end is fixed. The block is initially at rest at the position where the spring is unstretched (x = 0) when a constant horizontal force in the positive direction of the x axis is applied to it. A plot of the resulting kinetic energy of the block versus its position x is shown in Fig. 7-38.

    2. Relevant equations

    f= -kx
    k= 1/2 mv^2
    ui + ki = uf + kf

    3. The attempt at a solution

    i'm really confused about how to start this one :/
  2. jcsd
  3. Oct 7, 2009 #2


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    What are you supposed to find? I see no question there.
  4. Oct 7, 2009 #3
    oh, woops! i forgot to paste the rest :X

    The scale of the figure's vertical axis is set by Ks= 8.0 J. (a) What is the magnitude of ? (b) What is the value of k?

    sorry about that!
  5. Oct 7, 2009 #4


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    Your part (a) question still isn't complete.
    For (b), k is the maximum kinetic energy which the graph says is at distance 1 m.

    I don't think you can use your k = 1/2*m*v^2 directly. You'll have to think about the work being done by that force and the spring energy (have to look up the formula for the energy of a stretched spring). Give us a start on that and someone will help you if you need help.
  6. Oct 8, 2009 #5
    haha wow.. sorry about that
    the vector notation didn't actually paste properly

    and i really don't remember even learning the equation for a stretched spring, i'm just so lost
    it no longer really matters, because the homework was already due
    i just want to understand what's going on in the problem
    thanks for your help, though
  7. Oct 8, 2009 #6


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    Take a look here: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
    E = .5*k*x^2 where x is the distance the spring is stretched.
    The idea is that the work done goes into a combination of kinetic and spring energy:
    Fd = .5*k*x^2 + .5*m*v^2
    Might be worth doing even though too late to hand in. Problems like this will come back to haunt you on exams.
  8. Oct 9, 2009 #7
    i know you're right, i've just been so overwhelmed by this class lately
    i think i understand it now, thanks for your help :)
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