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: Basic Kinematics problem, why my method is invalid

  1. Dec 3, 2015 #1
    1. The problem statement, all variables and given/known data
    A motorist drives along a straight road at a constant speed of 14.4m/s. Just as she passes a parked motorcycle police officer, the officer starts to accelerate at 1.8m/s^2 to overtake her. Assuming the officer maintains this acceleration, determine the time it takes the police officer to reach the motorist (in seconds).

    2. Relevant equations
    vi^2 + 2ad = vf^2

    3. The attempt at a solution
    Here is what I did im curious why my method is incorrect.
    (14.4)t=d (speeder)
    vt=d (officer)
    v=at (officer)
    at^2=d (officer)

    1.8t^2 = 14.4 t
    t=0, 8

    Correct answer is 16

    Ok I realized that d = 1/2 vt^2
    this was my mistake, please show mathematically why I can't use these two equations to form d =vt^2

    vf =at (if vi=0)
    at^2 =d
  2. jcsd
  3. Dec 3, 2015 #2


    User Avatar

    Staff: Mentor

    vt = d only if velocity is constant over time. In this case it is not since there is acceleration involved. So you must turn to the full kinematic expression:

    ## d(t) = d_o + v_o t + \frac{1}{2} a t^2##
  4. Dec 3, 2015 #3
    Ok, thanks, great to know :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted