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Physics I (Connected Masses)

  1. Oct 14, 2012 #1
    1.I am stuck on this problem can anyone help me?

    A 20-kg object is subjected to three forces which produce an acceleration a = -8 m.s-2 i + 6.0 m.s-2 j on the object. Two of the forces are:
    F1 = 3.0 N i + 16.0 N j
    F2 = -12.0 N i+ 8.0 N j
    Find the third force.

    2. Relevant equations

    F = F1 + F2 + F3

    a = Fnet / m

    3. The attempt at a solution

    F1 = 3.0 N i + 16.0 N j
    F2 = -12.0 N i + 8.0 N j
    Fnet = -9 N i + 24 N j

    a = Fnet / m = -9 N / 20 kg i + 24 / 20 kg j = -0.5 i + 1.2 j
     
  2. jcsd
  3. Oct 14, 2012 #2

    tiny-tim

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    welcome to pf!

    hi eddiepi2012! welcome to pf! :smile:

    (try using the X2 button just above the Reply box :wink:)
    but that's not Fnet

    Fnet is the total force :wink:
     
  4. Oct 14, 2012 #3
    Ok so how do I find the total force I understand that I am searching for F3 but how do I find it based on the information given?? Any help will be greatly appreciated. Thanks tiny-tim.
     
    Last edited: Oct 14, 2012
  5. Oct 14, 2012 #4
    Thank you tiny-tim for welcoming me to PF. :)
     
    Last edited: Oct 14, 2012
  6. Oct 14, 2012 #5
    Re: welcome to pf!

    Thank you tiny-tim for welcoming me to PF. :)
     
  7. Oct 14, 2012 #6
    Re: welcome to pf!

    I believe this (-9 N i + 24 N j) is the Resultant of F1 + F2. Having this information how do I find F3?
     
  8. Oct 15, 2012 #7

    tiny-tim

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    hi eddiepi2012! :smile:

    (just got up :zzz:)
    first, use Fnet = ma to find Fnet

    then subtract F1 and F2
     
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