1. Limited time only! Sign up for a free 30min personal 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!

Stopping distance speed

  1. Mar 29, 2012 #1
    1. The problem statement, all variables and given/known data
    A driver carelessly ignores the reduced speed limit of 40.0 km/h in a school zone and continues at 65 km/h. Assuming a good reaction time of 0.80 s, how many more metres will it take him to stop than if he had reduced his speed? Assume 2000kg car has a constant braking friction of 12000N. (Must use work-energy theorem)
    va=11.11m/s
    vb=18.05ms
    Ff=12000N
    m=2000kg
    g=9.8m/s^2

    2. Relevant equations

    Ff=μFn
    Work done to an object = change in kinetic energy
    Wf=Ek
    μmgd=1/2mv^2
    d=v^2/2μg

    3. The attempt at a solution


    μ=Ff/Fn
    μ=12000N/(2000kg*9.8m/s^2)
    μ=0.61

    da=(11.11m/s)^2/2*(0.61)(9.8m/s^2)
    da=10.32m

    db=(18.05m/s)^2/2*(0.61)(9.8m.s^2)
    db=27.25m

    Δd=db-da
    Δd=27.25m - 10.32m
    Δd=16.93

    Therefore it takes the driver 16.93m longer to stop than if he slowed his speed.
     
    Last edited: Mar 29, 2012
  2. jcsd
  3. Mar 29, 2012 #2
    You must show us an attempt at a solution first before we are allowed to help you. First try to write an equation for distance if he had slowed down to 40km/h.
     
  4. Mar 29, 2012 #3
    i've tried the question using the knowledge i have but im not sure.
     
  5. Mar 29, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Start by stating the work-energy theorem.
     
  6. Mar 29, 2012 #5
    Hint: Use [itex]\frac{1}{2}[/itex]mv2=Fd
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook