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Velocity vs.Time graph

  1. Aug 4, 2008 #1
    I have question about the velocity vs. time graph about a slope that is below the x-axis. I was doing a problem about this:

    I have the x-axis as my time and y-axis as my velocity. Now at 3 seconds where velocity is -30 m/s to 6 seconds where velocity is 0 m/s, is acceleration "+" because it is in the same direction as velocity (since velocity is getting more positive and positive, -30, -25, -15,...)?
     
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  3. Aug 4, 2008 #2

    uart

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    Becuase t=6 seconds occurs later than t=3 seconds you should use v(3)=-30 as the *initial* velocity and v(6)=0 as the *final* velocity. Correct that and then you'll get the correct answer. Makes sense right?
     
  4. Aug 4, 2008 #3

    Hootenanny

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    Acceleration can be defined as the time derivative of velocity, which is the gradient on the velocity - time curve. So yes, if the gradient of the curve is positive (i.e. velocity is increasing) then the acceleration is positive.
     
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