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What's the deceleration of the car?

  1. Jan 31, 2009 #1
    1. The problem statement, all variables and given/known data
    A speed trap is set up with two pressure activated strips placed across a highway, 128 meters apart. A car is speeding along at 28.8 meters per second, while the speed limit is only 19.6 meters per second. At the instant the car activates the first strip, the driver begins slowing down. What minimum deceleration (magnitude only) is needed so that the driver's average speed is within the limit by the time the car crosses the second strip?

    2. Relevant equations
    Vf = Vi + a*t
    x = Vi*t + 0.5*a*t^2
    Vf^2 = Vi^2 + 2a*x

    Hint: Since the driver starts with a speed that is greater than the speed limit, his final speed will need to be less than the speed limit in order for the measured average speed to equal the speed limit.

    I have no idea how to start this.
  2. jcsd
  3. Feb 1, 2009 #2


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    Homework Helper

    Well, the average speed is given. And the initial speed. From that you can find the final speed.

    Knowing Vi and Vf, you should be able to use one of those formulas to find the acceleration.
  4. Feb 1, 2009 #3


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    You already have the distance to be traveled, and the beginning and final velocity. Plug and chug :tongue2:
  5. Feb 1, 2009 #4
    Since you know the speed limit and the distance, you can first calculate how long it will take to get from point A to B. Once you know this time it will reduce the number of variables you have to deal with.
  6. Feb 1, 2009 #5
    You're not given the final speed are you? I don't think you necessarily need to calculate it either.
  7. Feb 1, 2009 #6


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    Ah, I read the problem wrong, 19.6 is the average speed, not the final. :smile:
    But ok, how is your way quicker than finding Vf=19.6*2-28.8 and then continuing on?
  8. Feb 1, 2009 #7
    Sweeeet. Thank you. That helped a lot. So simple, yet I was making it difficult on myself.
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