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Homework Help: Lab help, can't get the same result as interactive physics

  1. Nov 13, 2011 #1
    1. The problem statement, all variables and given/known data
    A 15 kg block is against, but not attached to, a spring w/ k=150 N/m and pushed back a distance of .75 m. All of this is on an incline of 25 degrees, w/ coefficient of kinetic friction equal to .4. To top it off, there's a string pulling w/ a Tension of 20 N at an angle of 10 degrees above the incline. How far does the block move when released?

    2. Relevant equations
    Wnc=Kf-Ki+GPf-GPi+Sf-Si
    Wnc= nonconservative work
    K=kinetic energy
    GP=Universal Gravitational Potentiential Energy=mgh
    S=Spring Work=-1/2(k)(x^2)
    Fk=FN*mk
    Fk=friction force
    FN=normal force
    mk=.4


    3. The attempt at a solution
    Well, first I found the x and y gravitational forces, w/ the positive x-axis pointing up the incline. That's (147sin(25),147cos(25))=(62.125,133.227)N
    Then I found the same components for tension: (20cos(10),20sin(10))=(19.696,3.473)N

    FN=Fwy-Ty (the problem says nothing about the block leaving the incline, and in face he told us to put a frictionless block above it in IP to prevent this). So FN=133.227-3.473=129.754 N
    Fk=.4FN=51.9016 N

    WT=20cos(10)d=19.696d
    WFk=51.9016(cos180)(d)=-51.9016d

    Now I start using energy equations (initial and final position have its velocity at 0, so K is ignored)
    (15)(9.8)(dSin25)-15(9.8)(0)+0-(-1/2)(150)(.75^2)=19.696d-51.9016d
    62.125d+42.1875=-32.2056d (I was already suspicious here since the answer would clearly be negative)
    42.1875=-94.3306d

    ignoring a negative answer, I'd get d=.447 m
    IP (through Vector addition) gives me a displacement of .792m

    Now I didn't do the IP myself, a partner did, but I need to know if I did anything wrong here.
     
    Last edited: Nov 14, 2011
  2. jcsd
  3. Nov 15, 2011 #2

    Redbelly98

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    Homework Helper

    Welcome to Physics Forums.

    The potential energy stored in a spring is +(1/2) k x2 (no minus sign). The rest of your work seems okay.
     
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