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Homework Help: Unbalanced force

  1. Nov 5, 2011 #1
    1. The problem statement, all variables and given/known data

    A car has a mass of 1350 kg and an initial speed of 21 m s-1. Calculate the magnitude of the unbalanced force applied to the brakes that caused the car to come to a halt in 3 seconds.

    2. Relevant equations

    Force = mass x acceleration,
    = ( 1350 kg ) x ( -7 m s-2 ),
    = -9450 kg m s-2
    = -9450 N

    My question is this, is it ok to write the answer as a negative value of newtons, as above, because it doesn't seem right? I'm a little confused, any advice would be appreciated.

    3. The attempt at a solution
     
  2. jcsd
  3. Nov 5, 2011 #2

    Doc Al

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    Staff: Mentor

    Magnitudes are always positive. In this case, the negative sign just indicates the direction--that the acceleration and force were opposite to the velocity.
     
  4. Nov 5, 2011 #3
    so

    Force = mass x acceleration,
    = ( 1350 kg ) x ( -7 m s-2 ),
    = -9450 kg m s-2
    = 9450 N

    would be correct?
     
  5. Nov 5, 2011 #4

    Doc Al

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    Staff: Mentor

    Yes. Your answer would just be the magnitude--leave off any sign.
     
  6. Nov 5, 2011 #5
    thank you very much for your help.
     
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