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Homework Help: Energy Problem, Inclined plane, friction and spring Please help

  1. Dec 14, 2004 #1
    An object with mass 1 kg slides down a frictionless inclined plane of length 4 m set at an angle of 30 degrees. It then slides across a flat surface of length 1 m. While sliding across this surface the object experiences sliding friction with the coefficient of sliding friction being .6. It then strikes a spring with a spring constant of 1000 N/m. There is no friction present during the compression of the spring.

    What is the maximum compression of the spring, assuming that the object does not stop before hitting the spring?

    What is the speed of the object half way across the flat surface?

    My answers just don't seem right..... but here is my work maybe somebody out there can help...

    State 1 is the ramp, there i got
    F1= (mg)(sin30)=4.92 N

    State 2 is the flat surface:
    Etotal= Wc +Wnc
    Etot=(F1 x 1 m) + (-Ffr x 1)
    Et=(4.92 J) - (1kg x 9.81 m/s^2 x 1m x .6)
    Et= -.981 J

    State 3 is the spring:
    W=1/2 k x^2
    -.981 J= 1/2 (1000 Nm) x^2
    x= ? because you can't square root negative obviously, so I did somethign wrong and suposeably the correct answer is x=.1655 m and v=5.77 m/s.....but I want to figure out how to do the problem, becuase it seems im not doing it right please help someone thanks........

    Attached Files:

  2. jcsd
  3. Dec 14, 2004 #2


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    Homework Helper

    Here's your mistake. It should be:
    Etot=(F1 x 4 m) + (-Ffr x 1)

    The question says the inclined plane is 4m long, so F1 works through 4 m.
  4. Dec 14, 2004 #3
    I thought that at first, but as the the block is moving along the the flat surface doesnt that force act on the 1 m flat surface not the incline surface?
  5. Dec 14, 2004 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Think in terms of energy. The initial energy is potential: [itex]mgh = mgd sin\theta[/itex]. That's the energy the block has as it begins its horizontal slide.
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