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Motion + Spring problem

  1. Nov 16, 2011 #1
    1. The problem statement, all variables and given/known data

    A 16,000 kg F-18 (jet fighter) lands at 52 m/s on an aircraft carrier, its tail hook snags the cable to slow it down. The cable is attached to a spring with a spring constant of 60,000 N/m. How far does the spring stretch to stop the jet?

    2. Relevant equations

    f=-ks
    motion equations


    3. The attempt at a solution
    too hard to put on computer
     
  2. jcsd
  3. Nov 16, 2011 #2

    gneill

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    No effort, no help!
     
  4. Nov 16, 2011 #3
    i tried alot of different things, all the methods i tried didn't seem to work because it doesnt give neither time nor distance.
     
  5. Nov 16, 2011 #4

    SammyS

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    Show us what you tried, so we can do a better job helping you.
     
  6. Nov 17, 2011 #5
    i tried taking the velocity V=m/s and setting V=deltaX/deltaT. Then i solved delta T and got deltaT=deltaX/V. I then tried substituting this into the first motion equation vf=vi+adeltaT and solving for a, then used a in another motion equation V^2 = Vi^2 +2adeltaX after all this i think i got a =36.75 and t = 1.415 which i dont think is right
     
  7. Nov 17, 2011 #6

    gneill

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    You're right, it's not right :smile:

    As a spring stretches, the force it supplies increases. If the force is changing with distance, then so is the acceleration. If they are not constant, then you can't apply the 'usual' kinematic formulas that assume constant force or acceleration.

    Problems involving springs are often best approached from a conservation of energy point of view. Kinetic and potential energy get exchanged via the spring. Do you have the formulas for kinetic energy and spring potential energy?
     
  8. Nov 17, 2011 #7
    KE=1/2mv2 and Us=1/2k(deltaX)^2 correct? so do i set these equal to each other?
     
  9. Nov 17, 2011 #8

    gneill

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    Yes, you'll be setting them equal to each other. Do you know why?
     
  10. Nov 17, 2011 #9
    yeah i understand now, i just have trouble figuring out what equations to use. I just did it and i got distance is 26.85m. Thanks alot i really appreciate it
     
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