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Homework Help: Please Help problem due in 40 min

  1. Sep 11, 2007 #1
    1. The problem statement, all variables and given/known data
    Due to several previous unfortunate encounters with law enforcement, your car has been equipped with a GPS tracking device. You know that this device radios your current position to police headquarters at precisely hour intervals, but you don't know the exact time at which this occurs (i.e. it could be every hour on the hour, or every hour on the quarter hour, or something else). If the police can prove that you've driven faster than 100 km/hr at any point, then you are busted.

    You have been driving from State College to Sandy Springs, Utah (the latest hot Spring Break destination) along a perfectly straight road for several hours at a leisurely constant speed of 80 km/hr. Sandy Springs is only 46 km ahead. You realize that you can now speed up for the remainder of the trip, without getting busted.

    What is the maximum average speed at which you can finish the drive to Sandy Springs, with no chance of getting busted by your GPS tracker?

    2. Relevant equations

    At first, I tried using d = 1/2 (v0 +v)t since we don't know the acceleration. But we don't even know the time, so I tried using v^2=v0^2 +2ad, but we still don't know the acceleration. I'm starting to think that this problem requires more than 1 kinematics equation. But I don't know which ones. Can someone please help me?

    3. The attempt at a solution

    No idea.
  2. jcsd
  3. Sep 11, 2007 #2
    well, can vf be considered 100km/hr?
  4. Sep 11, 2007 #3


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    No complicated formulas here, just average speed. Worst case is that they check your speed just as you arrive. So you go at some velocity V for 46km. Takes time T=46km/V. The rest of the hour (1-T) was at 80km/hr. Compute an average speed for the last hour of the trip and see how big V can be before the average speed exceeds 100km/hr. I'm giving you more than the average number of hints since you have another kind of speed problem. Good luck in the 40min.
  5. Sep 11, 2007 #4


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    That's a guess, right? No, you can go faster. Not that I advocate violating the law.
  6. Sep 11, 2007 #5


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    Average speed over 1 hr <100km/h

    If it drove at v for the last 46km... what's the time for the last 46km... the rest of the hour it was driving at 80km/h

    Use (total distance)/(1 hr) = average speed <=100

    solve for v.
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