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!

Kinematic (suvat) equations?

  1. Dec 16, 2007 #1
    okay, this is simple and i feel ridiculous for asking, but i'm reviewing for my final and i do not remember how to do this type of problem..

    Runner A runs toward a flag pole at a constant speed of 5.0 m/s. Runner B begins to run toward the flagpole at 3.0 m/s^2 when Runner A is 15 m ahead. When will Runner B catch Runner A?

    I know I'll be setting two distance equations equal to each other, but beyond that I don't know what to do.
  2. jcsd
  3. Dec 16, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Have you come across any kinematic (suvat) equations?
  4. Dec 16, 2007 #3
    yeah, from kinematics:

    D: distance travelled (m)
    A: acceleration m/(s^2)
    t: time (s)
    D0: initial distance

    D = 0.5*A*t^2 + V*t + D0

    if you start your stop watch (t=0) when runner B starts to run, then runner A is already 15 m away. therefore:

    for runner A:
    DA = 5*t + 15

    for runner B:
    DB = 0.5*3*t^2

    DA: distance travelled by runner A
    DB: distance traveleed by runner B

    set DA = DB, you get a quadratic.
    solve the quadratic and the right answer is 5.24 s

    So it takes runner B be 5.24 s to catch up to runner runner A after he/she starts to run.

    If you start your stopwatch when runner A starts to run, then your time is 5.24 + 15/5 = 8.24 seconds

    Just double check my math please. hope this helps.
  5. Dec 16, 2007 #4
    Runner B needs to cover some extra distance so are you sure about the equation for runner B?
  6. Dec 16, 2007 #5
    I think he accounted for it with Runner A ( +15)
  7. Dec 17, 2007 #6
    Actually yes, I am sorry, I didn't see that.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Kinematic (suvat) equations?
  1. Kinematic Equations (Replies: 2)

  2. Kinematic Equations (Replies: 36)