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Homework Help: Finding acceleration in spring situations

  1. Nov 14, 2012 #1
    1. The problem statement, all variables and given/known data
    A duck of mass 40kg is being pulled by a spring with k = 900N/m. If μk = 0.6 between the duck and the road, find the acceleration of the duck given that the spring stretches 0.4m.

    2. Relevant equations
    Fspring = kx, where k = spring constant and x = stretch of spring
    Fnet = ma

    3. The attempt at a solution
    Fspring = kx
    F = 900N/m(0.6m)
    F = 540N

    That's about as far as I got before not knowing what to do next.
    Last edited: Nov 14, 2012
  2. jcsd
  3. Nov 14, 2012 #2


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    Those are two different forces, so give them different names.
    In which directions do they act?
    How do they relate to the acceleration of the duck?
  4. Nov 14, 2012 #3
    What exactly do you mean that they are two different forces? In the section that you quoted, isn't there only one force being calculated: the spring force?

    And p.s, I made a typing error. Where it says 0.4, it should be 0.6
  5. Nov 14, 2012 #4


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    OK, that's what made me think you were referring to two different forces.
    So, what is the frictional force acting on the duck (direction and magnitude)?
  6. Nov 14, 2012 #5
    I mixed some of the numbers apparently.

    Fspring = kx
    F = 900N/m(0.4m)
    F = 360N

    40kg x 9.8 = 392N

    Ff = μk(FN)
    Ff = 0.6(392N)
    Ff = 236N

    Fnet = Fa - Ff
    Fnet = 360N - 236N
    Fnet = 124N

    Fnet = ma
    Fnet/m = a
    124N / 40kg = a
    3.1 m/s2 = a

    Evidently that was a lot easier than I made it out to be :redface:
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