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Homework Help: Two forces (vectors)

  1. Mar 4, 2008 #1
    Two forces
    F1 = -8.20i + 4.30j and
    F2 = 7.10i + 4.60j
    are acting on a mass of m = 6.90 kg. The forces are measured in Newtons. What is the magnitude of the object's acceleration?

    i tried to add the vectors and using the f=ma to find acceleration but the answer is wrong.

    please help
    thnx
     

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    Last edited: Mar 4, 2008
  2. jcsd
  3. Mar 4, 2008 #2

    tiny-tim

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    Draw a picture!

    Hi mischaoc! Welcome to PF! :smile:

    It always helps to draw a diagram first (just roughly), to see what's going on.

    Draw the i and j axes, and three lines with arrows on to represent the two forces and a guess as to the acceleration.

    Do you know the relationship between those three lines?
     
  4. Mar 4, 2008 #3
    Have you neglected to include the force of the mass acting downwards due to gravity? I know that is a mistake that my sister has made when completing similar problems.
     
  5. Mar 4, 2008 #4

    tiny-tim

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    nice diagram! and colour-coded … i'm impressed!

    Hi mischaoc!

    From the way you've drawn the diagram, it seems to me that you've worked out what the rule is. :smile:

    What's worrying you?

    Show us your working, and the right answer (actually, you should have done that originally).

    (and ignore mike's sister - I always ignore mine! :smile:)
     
  6. Mar 4, 2008 #5
    my work

    i added the vectors and gor F3=-1.1i+8.9j
    then i calculated the magnitude of
    F3=[tex]\sqrt{(-1.1)^2+(8.9)^2}[/tex]=8.9 N

    i found the angle [tex]\alpha[/tex]=tan-1 [tex]\frac{8.9}{-1.1}[/tex]
    so [tex]\alpha[/tex]=97.5
    using the F=ma formula:
    8.9*cos(97.5)=6.9*a
    a=-0.168
    but it's wrong.
     
  7. Mar 4, 2008 #6

    tiny-tim

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    Aha! … now, that's not right, is it? :frown:

    You see - that is why the forum rule is that you show what you've done!

    Is it ok now? :smile:
     
  8. Mar 4, 2008 #7
    it's F3=8.98 N
    but it is still wrong
     
  9. Mar 4, 2008 #8

    tiny-tim

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    Actually, I make it 8.968 (or 8.97).

    Why did you put the angle in?

    (Are you thinking that j is vertical, and that somehow the weight is involved? That's not what the question says.)

    You have the size and direction of the force.

    So you divide by m to get the size and direction of the acceleration. That's all!

    Don't make it more complicated! :smile:
     
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