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Stopping Distance Problem

  1. Sep 9, 2011 #1
    I was hoping you could help me with a stopping distance problem?

    Reaction Time=1.5 seconds
    Coefficient of Friction= .7
    Distance=506 Feet
    Ending Speed= 30mph
    Starting Speed?

    I was wondering you could help me figure out the original speed the car was traveling if after 506 feet braking and the other information above, the car was traveling 30mph.

    Disclaimer* This is not homework. Someone claims i cut them off at work today and im trying to prove them wrong by finding out their original speed.

  2. jcsd
  3. Sep 9, 2011 #2


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    Staff: Mentor

    They locked up their brakes for 500 feet?
  4. Sep 9, 2011 #3
    no here's what happened. I pulled out of an intersection and then proceeded to pull over at the next stop to pick up a passenger. (im a bus driver) The distance between where i pulled out and then came to a stop was 506 feet. As soon as i got to the stop a woman laid on the horn and blew by me. She then called the company i work for and filed a complaint saying i cut her off. I just want to know theoretically how fast would she have been going if she was on the brakes for the full 506 feet and still couldnt stop and ending up passing me at 30mph.

    She definitely wasnt paying attention and didnt see me stop, this is more just to satisfy my curiosity then anything else. Hopefully all of that makes sense
  5. Sep 9, 2011 #4
    It would depend on how hard she was on the brakes. You can set up an equation where the work done by friction equals the change in kinetic energy of the car. If the wheels are locked up you may get a coefficient of friction of 0.7. But I doubt that would be the case because the car would have stopped sooner. I have performed the calculations. The results say, even if you take the 1.5 second delay as part of the 506 feet, that the speed the car was going was very, very fast which I doubt because few cars will go that fast.

    An average car will stop from 60 mph to zero in about 125 feet. From 80 mph it takes about 220 feet to stop.
  6. Sep 9, 2011 #5
    Plus you have to consider ABS
  7. Sep 10, 2011 #6


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    Science Advisor
    Homework Helper

    ABS only works if you first press the brake pedal. Looking out of the windows while driving also helps.

    I suspect lack of one or both of those activities was most likely to be the cause of the "problem".

    Certainly in the UK, if you want to pick a fight with bus that is indicating it is stopping or pulling out from a stop, you lose. It's a bus. That's what they do.
  8. Sep 10, 2011 #7
    i was only talking about the physics. My guess? He stopped way too short, the other driver wasn't paying attention. Both are at fault, both should get over it. The only reason I side with the OP is that the woman brought it to the attention of his place of work. If no one was hurt, and no property was damaged, get the heck on with your life, eh?

    BTW, I hope this is just for your own satisfaction, yz2009, because nobody is going to take your calculations seriously...
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