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Finding acceleration of an object (with friction)

  1. Oct 15, 2011 #1
    1. The problem statement, all variables and given/known data
    A 1,300-N crate is being pulled across a level floor at a constant speed by a force of 310 N at an angle of 20.0° above the horizontal. If coefficient of kinetic friction is 0.207, find the acceleration of the crate.


    2. Relevant equations
    Fk= (uk)(N)
    F=ma


    3. The attempt at a solution
    Fx= 310cos20
    Fy= 310sin20

    N=1300-310sin20

    Fk=(uk)(N) = (0.207)(1300-310sin20) = 247.15 N

    sumFx= Fx-Fk =310cos20-247.15 = 44.15 N

    F=ma=44.15
    a=44.15/1300 =0.0340 m/s^2

    Answer says a=0.331

    Can someone point out what I did wrong? Thanks
     
    Last edited: Oct 16, 2011
  2. jcsd
  3. Oct 15, 2011 #2

    SammyS

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    Doesn't gravity exert a force in the y direction, contributing to Fy ?

    The Fy you have is downward. Do you want that to be negative ?
     
  4. Oct 16, 2011 #3
    im not sure if I quite follow. Yes, force of gravity (1300N) is exerted in the y-direction:

    F1y= 310sin20
    N=1300-310sin20
    sumFy=N+F1y-1300N=0

    I'm not sure what is wrong. Can someone help me out?
     
  5. Oct 16, 2011 #4

    Doc Al

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    The applied force is downward:
     
  6. Oct 16, 2011 #5
    sorry that should be above the horizontal
     
  7. Oct 16, 2011 #6

    Doc Al

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    1300 N is the weight of the crate, not its mass.
     
  8. Oct 16, 2011 #7
    so, how would i be able to find the acceleration?
     
  9. Oct 16, 2011 #8

    Doc Al

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    Just like you tried to do, only first find the mass of the crate. How do weight and mass relate to each other?
     
  10. Oct 16, 2011 #9
    Got it, thanks. W=mg...find m using weight and use f=ma to find acceleration.
     
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