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Introductory kinematics word problem

  1. Jul 4, 2009 #1
    1. The problem statement, all variables and given/known data

    A competitor is aiming to complete a 1500m wheel-chair race in less than 4.0 minutes. After moving at a constant speed for exactly 3.5 minutes, there were still 240m to go. What must his acceleration before the remaining distance if he were to finish the race on time?

    d1= 1260m [forward]
    d2= 240m [forward]
    t1= 210s
    t2= 30s

    Required: v1, v2 possibly?, and a

    2. Relevant equations

    v1 = d/t
    and d = v1*(t+a(t)^2)/2 ?

    3. The attempt at a solution


    d = v1*(t+a(t)^2)/2
    a= (2d/t^2)-v1*t
    a= (2*240m/30s^2) - 6m/s*30s
    a= something horribly incorrect

    the answer at the back of the book is 0.13m/s^2. im guessing that im not approaching the second part of the question correctly. i tried it again with an equation using v2, but im still getting weird answers.

    help please !

    also, im sorry i dont know how to use latex :/
  2. jcsd
  3. Jul 4, 2009 #2


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    Welcome to PF!

    Hi shaools! Welcome to PF! :smile:

    (don't bother about LaTeX for something like this … the SUB and SUP tags are fine, and they save a bit of server space also! :wink:)
    hmm … you're messy with brackets, which leads you to make mistakes

    that middle line is wrong. :smile:
  4. Jul 4, 2009 #3

    i managed to screw up the initial equation twice in that post. there's no way to edit posts is there?

    i meant to post this equation for displacement and acceleration:

    http://id.mind.net/~zona/mstm/physics/mechanics/kinematics/EquationsForAcceleratedMotion/Origins/Displacement/Image78.gif" [Broken]


    d = v1*t + (a*t^2)/2
    a = 2d/(t^2) - v1*t

    and then when i plug my values in, i get funky answers.
    i think i might just be using the wrong equation, or i might be plugging in incorrect values. im really not sure.

    thank you for checking tho!
    Last edited by a moderator: May 4, 2017
  5. Jul 4, 2009 #4


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    Nope … still wrong :redface:

    Write it out step by step … don't try to do any of it in your head!
  6. Jul 4, 2009 #5
    ah, bested by bedmas

    lol let me attempt this once more...

    a= [2(d - vt)]/ t^2 ?
  7. Jul 4, 2009 #6
    a = [2(240m - 6m/s*30s)] / 30^2
    a = 0.13m/s^2

    yup :).

    thank you mr. fish !
  8. Jul 4, 2009 #7


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    :tongue2: blubblelubbleglubblelubblephlrrrrrrp! :tongue2:
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