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Homework Help: Exercise in Net Force

  1. Sep 13, 2007 #1
    1. The problem statement, all variables and given/known data
    Two forces, F1 and F2, act on the 7.00-kg block shown in the drawing. The magnitudes of the forces are F1=65.9 N and F2=22.7 N. Take the positive direction to be to the right. Find the horizontal acceleration of the block, including sign.

    http://img522.imageshack.us/my.php?image=forcett9.jpg


    2. Relevant equations

    [tex]\Sigma[/tex]F = ma or a = [tex]\Sigma[/tex]F / m

    [tex]\Sigma[/tex]F - Net Force (F1 + F2 = Net F)
    m - mass
    a - acceleration


    3. The attempt at a solution

    F1 = 65.9N
    F1 * Sin = 61.9N
    F1 * Cos = 22.6N

    F2 = 22.7 N
    F2 Opposite Force = -22.7 N

    (-) 22.7 N + (+) 22.54 N = 7 kg*a
    (-).16 N = 7 kg*a
    (-).16 N / 7 kg = a
    (-)0.023 m/s[tex]^{2}[/tex]


    I'm not quite sure what I'm doing wrong as when I try to use this answer it is marked as incorrect so any help would be most appreciated.
     
  2. jcsd
  3. Sep 13, 2007 #2

    learningphysics

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    -0.023 m/s^2 was marked as incorrect?
     
  4. Sep 13, 2007 #3
    Correct; my answer is marked as incorrect but I'm not sure why as it seems to be the only answer I can arrive at using the formula taught to me both in my class and in my recitation lab.
     
  5. Sep 13, 2007 #4

    learningphysics

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    Did you post the question exactly as it is? Did you maybe mix up F1 and F2?
     
  6. Sep 13, 2007 #5
    Directly as is. I was very careful to type everything word, and number, for word.

    Is there perhaps an error with the homework you think?
     
  7. Sep 13, 2007 #6

    learningphysics

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    yeah I think so... I suspect maybe there's a coefficient of friction that's supposed to be in the problem... but they left out...

    Unless the program is sensitive about significant figures or something?
    Maybe -0.0230m/s^2 will work? Not sure...
     
  8. Sep 13, 2007 #7
    I tried the extra zero at the end of -0.0230 m/s^2 and it took the answer, finally, so I think the program is just that sensitive. Thank you so much for your help. I was starting to stress so much I don't think I would have thought to keep adding on significant numbers.
     
  9. Sep 13, 2007 #8

    learningphysics

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    No prob. Glad it went through. :smile:
     
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