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Homework Help: Acceleration/veloctiy/displacement/time question

  1. Sep 3, 2009 #1
    Ok i just started gr 11 physics, and im wondering how would you tackle this problem

    In a long distance race, Michael is running at 3.8 m/s and is 75 m behind robert, who is running at a constant velocity of 4.2 m/s. If Michael accelerates at 0.15 m/s2, how long will it take him to catch Robert?

    Im assuming in this question you need 2 equations to find the unknown, I don't know how to make an equation with robert being ahead 75m.. :(

    Possible formulas that could be used are

    V = d/t

    d = v1 x t + 1/2(a)t2 <-- (time squared)

    a = vf - vi / t <--- final velocity - intial velocity

    d = vi + vf /2 (t)

    Please keep things simple ^^ as ive just started gr 11 physics :)

    Thanks for the help
  2. jcsd
  3. Sep 3, 2009 #2


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    Homework Helper

    Welcome to PF.
    You need two equations to solve this problem.
    In time t Robert move a distance x. The in the same time what is the distance moved by Michael when he meets Robert?
  4. Sep 3, 2009 #3
    Thanks for the quick reply

    hmm... so the first equation would be t = d / 4.2m/s

    than the 2nd part... would be d = (3.8m/s)(t) + (1/2)(0.15m/s2)(t2)

    is this right so far?
  5. Sep 3, 2009 #4


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    Homework Helper

    The first one is
    d = 4.2 m/s*t
    And in the second one the displacement is d + 75 m.
  6. Sep 3, 2009 #5
    Ohhh... Thanks very much
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