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!

Homework Help: Many failed attempts on this displacement/acceleration/velocity problem. please help

  1. Apr 15, 2012 #1
    1. The problem statement, all variables and given/known data

    An unmarked police car traveling a constant 80 km/h is passed by a speeder traveling 125 km/h.
    Precisely 3.00 s after the speeder passes, the police officer steps on the accelerator; if the police car's acceleration is 2.40 m/s^2, how much time passes before the police car overtakes the speeder after the speeder passes (assumed moving at constant speed)?

    2. Relevant equations

    ΔX (speeder) = V(speeder)T → (125km/h) (1m/s / 3.6km/h) (T) = (34.72222T)m

    ΔX (police) = V (police)(3.00s) →(80km/h)(1m/s / 3.6km/h) (3.00s) = 66.666667m
    ΔX (police during accelerated motion) = V (t-3.00) + 1/2(2.40m/s^2)(t-3.00)^2
    Total ΔX (police) = 66.666667 + 66.666667(t-3.00)+1/2(2.40)(t-3.00)^2

    ΔX (speeder) = ΔX (police)

    3. The attempt at a solution


    Were all my answers but all are wrong. Please let me know what I did wrong. Thank you so much in advance!
  2. jcsd
  3. Apr 15, 2012 #2


    User Avatar

    Re: many failed attempts on this displacement/acceleration/velocity problem. please h

    You know that your third and fourth answers can't possibly be right, because the police car does not even start to accelerate until three seconds have passed.

    Use the basic kinematic equations, and write down an equation for the speeder and another for the cop.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook