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Homework Help: Reaction time, beraking decelerationPlease help

  1. Apr 27, 2008 #1
    reaction time, beraking deceleration..Please help:)

    1. The problem statement, all variables and given/known data

    A pedestrian steps on to the road while an approaching car is travelling at 30 km/h. If the driver's reaction time is 0.3 s and the braking deceleration is 4.5 m/s, calculate a) the stopping distance; (b) the stopping time

    2. Relevant equations

    v2=u2 + 2as?

    3. The attempt at a solution

    I tried to work out the stopping distance by converting u=30 km/h to m/s then i used the equation s=(u+v)*t/2 but im not familiar with all this reaction time stuff? and also i know the final speed is 0 but how can i find all the information with the reaction time and the breaking deceleration?
    Could someone please help me:) I would really appreciate it! Thankyou very much
  2. jcsd
  3. Apr 27, 2008 #2
    When the pedestrian steps into the road, the car will continue to travel at 30 km/h until the driver hits the breaks, which in your case is 0.3 seconds (very fast if you ask me :). The stopping distance is then the combined distance of this and how far it takes for the car to decelerate to a full stop once the driver hits the breaks.
  4. Apr 27, 2008 #3
    Thankyou so much! that realli helps but um i dont know how to find the distance of how far it takes for the car to decelerate to a full stop.. umm do i need to find the decelleration first so i have v,u,a and then i can use the equation v2=u2+2as?
  5. Apr 27, 2008 #4
    Well, when trying to find equations I always try to look at what I have and what I want, and see if I can find one so that the one variable I want is the only unknown.

    In this case you have the initial speed u and the final speed v, and you have the acceleration a. What you want is the distance s.
  6. Apr 27, 2008 #5
    oh woops hehe i have it now, thankyou very much!!!
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