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Homework Help: Force at downward angle

  1. Oct 4, 2006 #1
    A box of books weighing 320 N is shoved across the floor of an apartment by a force of 430 N exerted downward at an angle of 35.6° below the horizontal. If the coefficient of kinetic friction between box and floor is 0.57, how long does it take to move the box 3.90 m, starting from rest?

    I found n to be 260.19 but now i don't know where to go with it. What formula should I use? I can't find one that includes time.
     
  2. jcsd
  3. Oct 4, 2006 #2

    Doc Al

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    Staff: Mentor

    What is "n" and how did you find this?

    Hint: To calculate the normal force you must include all vertical forces acting on the books.
     
  4. Oct 4, 2006 #3
    I personally am having trouble with this exact same question only with different numbers.
    I have found so far, the equations:

    m*a[x] = F[applied]*cos(Θ) - force of friction.
    m*a[y] = F[normal] - F[gravity] - F[applied]*sin(Θ)

    and

    m*a = F[applied]*cos(Θ) - mu *(m*g + F[applied]*sin(Θ))

    However I continually get the wrong answer no matter how I go about it and have no clue what I'm doing wrong, or even if those equations are right.

    Any help would also be appriciated by me, greatly.
     
  5. Oct 5, 2006 #4

    andrevdh

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    Homework Helper

    Stonie, your equations are all correct so post the calculation.
     
  6. Oct 8, 2006 #5
    I solved those equations but I still don't understand how to find time from them. Help please?
     
  7. Oct 9, 2006 #6

    Doc Al

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    Staff: Mentor

    Once you've solved for the acceleration, it becomes a kinematics problem.
     
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