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Homework Help: Work-Energy principle and spring force

  1. Jun 10, 2015 #1
    A block lies on a horizontal frictionless surface, and the spring constant is 50 N/m. Initially, the spring
    is at its relaxed length and the block is station ary at position x = 0 .Then an applied force with a constant magnitude of 3 N pulls the block in the positive direction of the x axis,stretching the spring until the block stops. When that stopping point is reached, what are (a) the position of the block, (b) block's position when its kinetic energy is maximum

    Ws(work done by spring force) = -Wapp(work done by applied force) , Fs(spring force) = -kx , Ws = -1/2 kx2

    I tried to solve the solution by using the concept , that because our applied force is of constant magnitude and because the block stops after some displacement x , then at that point Fapp = -Fs
    ==> Fapp = -(-kx) ==> 3= 50x ==> x = 3/50 = 0.06
    But the answer and method according to the solution manual is different, it is given below
    This is a situation where Eq Ws = -Wapp , applies, so we have

    Fx = 1/2 kx2
    ==> 3 x = 1/2 (50 )x2
    which (other than the zero) gives x = (3.0/25) m = 0.12 m.
    What is wrong with my logic, the second part of the question is clear to me but i am providing its answer according to the solution manual because it seems relevant part first and might help in understanding part first better.

    With Kf = K considered variable and Ki = 0, Eq. 7-27 gives K = Fx – 1/2kx2. We take
    the derivative of K with respect to x and set the resulting expression equal to zero, in
    order to find the position xc which corresponds to a maximum value of K:
    xc = F/k= (3.0/50) m = 0.060 m.
  2. jcsd
  3. Jun 10, 2015 #2


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    Homework Helper

    Welcome to PF!

    The block stops (momentarily) means that its velocity is zero at that moment. But it does not mean that the net force is zero; the acceleration is zero instead.
    When only a constant force acts on the block, connected to the spring, it will perform SHM when no dissipative force is present. When it has maximum kinetic energy, its acceleration is zero, and when the kinetic energy is zero, the magnitude of acceracion is maximum.
    You can do a little experiment: gravity also means a constant force. Hang some weight on an elastic spring. Holding the other end of the spring, let the weight fall : the weight will vibrate between a maximum and minimum height.
    Last edited: Jun 10, 2015
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