1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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!!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Reaction time, beraking decelerationPlease help
  1. Reaction time (Replies: 2)

Loading...