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Finding acceleration

  1. Nov 4, 2007 #1
    1. The problem statement, all variables and given/known data
    A 900 N crate is being pushed across a level floor by a force of 313 N at an angle of 25 degree above the horizontal. The coefficient of kinetic friction between the crate and the floor is 0.17. Acceleration of gravity is 9.81 m/s^2

    http://img233.imageshack.us/img233/3717/31nk3.jpg

    What is the acceleration of the box in m/s^2?


    2. Relevant equations

    F = ma


    3. The attempt at a solution

    Fg = 900 N
    mass of crate = 91.743 kg
    U = 0.17
    g = 9.81 m/s^2
    theta = 25
    Fn = 900 + 132.2795
    Ff = 0.17 * Fn = 175.4875

    313cos 25 = 283.674 N
    313sin 25 = 132.2795 N

    Fx = ma
    283.67 = 987.743a
    a = 3.201 m/s^2

    I didn't know whether or not to incorporate the y direction forces
     
    Last edited: Nov 4, 2007
  2. jcsd
  3. Nov 4, 2007 #2

    Doc Al

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    Staff: Mentor

    Looks like you forgot to include friction.
     
  4. Nov 4, 2007 #3
    Is the box moving up or down? What would this infer about acceleration?
     
  5. Nov 4, 2007 #4
    The box is moving left not up or down. Since the force is applied at an angle, the acceleration is not as great if applied directly horizontal
     
  6. Nov 4, 2007 #5
    F = UFN
    Friction = 0.17* (900 + 132.2795)
    Friction = 175.4875 N
     
  7. Nov 4, 2007 #6

    Doc Al

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    Staff: Mentor

    Good. Now recalculate the total force in the x-direction. (Which way does friction act?)
     
  8. Nov 4, 2007 #7
    Friction act against the block, therefore, total force in x direction:

    283.67 - 175.4875 = ma
    111.1825 = 91.7431a
    111.1825 / 91.7431 = a
    a = 1.211889 m/s^2

    did I do that right?
     
  9. Nov 4, 2007 #8

    Doc Al

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    Staff: Mentor

    Looks good. (Round off your answer to a reasonable number of significant figures.)
     
  10. Nov 4, 2007 #9
    I plugged it into the site where we're suppose to turn in our answers and it says it wrong. Hmmm
     
  11. Nov 4, 2007 #10

    Doc Al

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    Staff: Mentor

    arithmetic error!

    Oops... double check that subtration. (Sorry for not catching that earlier.)
     
  12. Nov 4, 2007 #11
    yay it worked thanks so much! The answer is 1.179 m/s^2
     
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