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Acceleration from 2 Force Vectors

  1. Feb 9, 2008 #1
    1. The problem statement, all variables and given/known data


    Two forces, F1 and F2, act on a 2.00 kg object where F1 = 30.0 N (on the x-axis) and F2 = 10.0 N. A 60 degree angle is between them. Find the magnitude and direction of the acceleration.

    http://www.webassign.net/pse/p5-15.gif (ignore figure a)

    2. Relevant equations
    F=ma
    a=sqrt. (ax^2 + ay^2)


    3. The attempt at a solution
    I figured out that the magnitude is a=sqrt. (ax^2 + ay^2)
    which is equal to 17.67

    however i cant get the correct direction! i thought it would be theta = arctan (2.5 /17.5)= 8.13 but it keeps saying its not the right answer! help
     
    Last edited: Feb 9, 2008
  2. jcsd
  3. Feb 9, 2008 #2

    Doc Al

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    Staff: Mentor

    The angle should be arctan (a_y/a_x).
     
  4. Feb 9, 2008 #3
    General rules for these problems:

    Use units throughout.
    Combine the two forces into a single one.
    Show all work.
     
  5. Feb 9, 2008 #4
    i did that and it didnt come out right
     
  6. Feb 9, 2008 #5

    Doc Al

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    Where did you get the numbers 2.5 and 17.5? What are they?
     
  7. Feb 9, 2008 #6
    2.5= ay
    17.5= ax

    together they are the acceleration = 17.67 (im 100% sure)

    i got them by using this technique
    Fx= 30cos(0)+10cos(60)= 35
    A=F/m A= 35/2 A= 17.5

    Fy= 30sin(0)+10cos(60)= 5
    A=F/m A= 5/2 A= 2.5
     
  8. Feb 9, 2008 #7

    Doc Al

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    Redo your calculation of Fy.
     
  9. Feb 9, 2008 #8
    oops no i used sine just didnt type it out right here
     
  10. Feb 9, 2008 #9

    Doc Al

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    But [itex]10 \sin(60) \ne 5[/itex]
     
  11. Feb 13, 2008 #10
    why is it the cos(0) and sin(0)
     
  12. Feb 13, 2008 #11

    Doc Al

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    Because the angle that the 30 N force makes with the x-axis is 0. (That force is along the x-axis.)
     
  13. Feb 13, 2008 #12
    how do i find out the angle is i have the magnitudes!?!?
     
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