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Physics - proving the stopping distance of a car

  1. Apr 24, 2016 #1
    1. The problem statement, all variables and given/known data
    an accident occurs up ahead on the highway. a driver travelling at 120km/h [e] reacts in 0.20s and applies the brakes causing an acceleration of 8.0m/s2 [w]. show that the stopping distance is 76 m.
    what am i doing wrong? i changed the acceleration to match the directions, so i made it negative. but i am not getting the right answer.

    2. Relevant equations
    d = vi * t + 0.5(a * t^2)

    3. The attempt at a solution
    i attempted this by using:

    d = (120km/h / 3.6[e])(0.20s) + 0.5(-8.0m/s[e] * 0.20s^2)

    d = 6.5m?
     
  2. jcsd
  3. Apr 24, 2016 #2

    SteamKing

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    The problem is you have assumed that the car goes from 120 kph to 0 kph in 0.2 s, which is not what the problem states. The driver takes 0.2 s to press the brake pedal after he sees the accident ahead of him.

    You should pick another SUVAT equation which relates distance, acceleration,and initial and final velocity.
     
  4. Apr 24, 2016 #3
    So, I just have another question, in this question the final velocity would be at a stop, so 0 m/s correct? and also would i have to find the distance traveled before pressing on the brakes and add it to the distance it took while slowing down?
     
  5. Apr 24, 2016 #4

    SteamKing

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    Yes and yes.
     
  6. Apr 24, 2016 #5
    thank you, your answers have been very helpful and i am very grateful!
     
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