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Momentum/ Spring Question

  1. Jun 4, 2010 #1
    Hi,

    I have just tried a momentum question involving springs. I got the wrong answer and the method used in the solution is different, but i still do not see what i did wrong.

    The question is: a railway engine 6.2*10^4 and a carriage 4*10^4 have coupled. They have hit a buffer spring (to stop them) from an initial speed of 0.15m/s. Assuming the buffer behaves like a spring of stiffness 320kN/m calculate the maximum compressiion of the spring.

    The solution given is:

    Kinetic energy of train :

    0.5 × 10.2 × 104 × 0.152 = 1150 J

    0.5* F e
    = 0.5× (k e) × e = 0.5 × k e^2
    0.5 × 320 × 103 e2 = 1150 1
    gives compression e = 8.47 × 10^-2 m

    My Solution
    F=Change in MV/Change in time F= Ke

    so:

    MV/T = Ke


    s= ((u+v)/2)t Therefore : t=(40/3)e

    so Mv= 40/3(Ke^2)

    And i calculate e=0.059 m

    But this is different to the solution given, why does my method not work?

    Thanks
     
  2. jcsd
  3. Jun 4, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Δ(MV)/Δt would give you the average force, not the maximum force.


    The acceleration is not uniform, thus that kinematic formula will not apply.
     
  4. Jun 4, 2010 #3
    Ok thank you very much for you assistance.
     
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