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Homework Help: Kinematics / Acceleration Problem

  1. Dec 9, 2007 #1
    1. The problem statement, all variables and given/known data

    Q1. In a 100m sprint the athlete can be assumed to accelerate linearly for the first 60m (which takes 7s to cover). For the remaining 40m the running speed is constant.
    a) Determine the acceleration over the initial 60m. (5 marks)

    b) Draw a graph of
    i) acceleration v time (2 marks)
    ii) speed v time (2 marks)
    iii) distance v time (2 marks)

    c) Determine the time taken to complete 100m both graphically and numerically. (4 marks)

    2. Relevant equations

    s = ut + 0.5at^2

    s = displacement

    t = time

    a = acceleration

    v = u + at

    v = velocity

    u = initial velocity

    t = time

    s = 0.5(u+v)t

    3. The attempt at a solution

    Acceleration = 2.163 m/s^3

    Velocity = 15.141m/s


    My problem is when i use other kinematic equations i dont get back to the same values as given in the problem.

    when using formula

    V^2 = U^2 +2as

    i get a different velocty

    and when i use formula

    s = 0.5at^2

    i get a different displacement.


    So have i solved this corrent?

    Any advice on what is the corrent formula or what is the correct answer would be most appreciated... thanks
  2. jcsd
  3. Dec 9, 2007 #2


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    Homework Helper

    i think you may have made a numerical calculation error.... check numbers again
  4. Dec 9, 2007 #3
    Hey I'm not that good at physics or anything but i'll give it a go....

    D=ut + .5 a t^2
    60= 0 + 1/2 a 7^2
    120 = 49a
    a = 2.45 m/s....only slightly different to what you got.
  5. Dec 9, 2007 #4
    Yes, i think you are 100% corrent, i am just going over all the Kinematic equations again.

    I belive what i have done is a pre-school error.

    I have done 0 * 7 and managed to get 7 =) which its supposed to be 0.

    Think its all fixed, thanks...
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