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Forces help!

  1. Sep 30, 2007 #1
    1. The problem statement, all variables and given/known data

    Three forces act on a particle that moves with a constant velocity v = 2 m/s x – 7 m/s y . Two of the forces acting on this particle are F1 = -10.0N x + 9.0N y + 4.0N z and F2 = = -1.0N x + 4.0N y + 2.0N z . What is the magnitude of the third force?


    2. Relevant equations

    Fnet=ma?

    3. The attempt at a solution

    I'm actually not even sure what the question is asking to be honest....
     
    Last edited: Sep 30, 2007
  2. jcsd
  3. Sep 30, 2007 #2

    Dick

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    F=ma, indeed. So a=0 since it's moving with constant velocity, so the sum of the forces is zero. What's the third force? Once you have it, figure out it's magnitude.
     
  4. Sep 30, 2007 #3
    So I just go:

    Fx: -10-1+____=2
    Fy: 9+4+___=-7
    Fz: 4+2+__=0
    Where the blanks are the third force?
     
  5. Sep 30, 2007 #4

    Dick

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    No. The velocity is CONSTANT. So the acceleration is ZERO. Set all of your equations equal to zero.
     
  6. Oct 4, 2007 #5
    All of which equations? I get that Fnet = 0 because there is no acceleration, but how do I relate that to velocity? or to the components of the other forces?
     
  7. Oct 4, 2007 #6

    Dick

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    All of THESE equations. The velocity doesn't matter since it's constant. The equations above give you the components of Fnet. Which should be zero, so replace the 2 and -7 with 0. Now figure out what goes in the blanks to give you the third force.
     
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