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Homework Help: Am I approach this question right? finding acceleration

  1. Mar 31, 2008 #1
    Am I approach this question right?......finding acceleration

    1. The problem statement, all variables and given/known data

    A tractor applies a force of 1.3kN to the sled, which has a mass of 1.1x10^4 kg. At that point, the co-efficient of kinetic friction between the sled and the ground has increased to .8. What is the acceleration of the sled?


    2. Relevant equations


    3. The attempt at a solution

    Given


    M = 11,000kg
    Applied Force = 1300 N , let if be Fa
    Kinetic Friction co-efficient = 0.8 , let it be uK



    Legend


    let Fa = applied force
    let Ff = force of friction
    m = mass
    a = acceleration


    Finding force of friction


    Ff = uk*Fn
    = (0.8)* (11000 kg x 9.81m/s^2)
    = 86 328 N



    Calculating Acceleration


    F=ma
    Fa + Ff = ma
    a = ( Fa + Ff ) / m
    = ( 1300 N + 86 328 N) / 11 000 kg
    = 7.9 m/s^2




    I tried to do this question literally 10 times now. I keep getting this answer, but know it's wrong. Maybe my approach isn't correct, or I am missing a negative sign somewhere. Please help.
     
  2. jcsd
  3. Mar 31, 2008 #2
    so it should be

    Fa - Ff = ma


    ?
     
  4. Mar 31, 2008 #3
    yes
    Applied force(Fa) - frictional force(Ff) = mass times acceleration

    but don't confuse the 'a' in 'Fa' with acceleration
    - its just being used to denote 'applied', the symbols used are a little confusing and they should have made more effort to avoid that.
     
  5. Mar 31, 2008 #4


    Fa - Ff = ma

    1300 N - 86328 N = m * a

    therefore acceleration = (1300N - 86328N) / 11 000 kg

    = -7.729 m/s^2



    but in the book, the answer is 0.61 m/s^2
     
  6. Mar 31, 2008 #5
    anybody?

    this question is really bothering me............ it's intro physics, and I'm struggling already.
     
  7. Mar 31, 2008 #6
    Perhaps the book is wrong because I attained the same answer without reading the thread until now.
     
  8. Mar 31, 2008 #7
    lol....wow

    I've been beating my self all day over this question.
     
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