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Homework Help: Kinemetics Question - theoretical subsitution

  1. Oct 21, 2011 #1
    1. The problem statement, all variables and given/known data

    This is for personal study and as im on study leave its a bit hard to contact a teacher.

    This is the question; (the circled answer was the teachers answer)

    2sba5pl.jpg

    I'm unsure as to why you would individually square the times before subtracting as the actual time is (t2-t1) substituted into time on the equation, squaring before will result in a larger value (small value for acceleration though). It is possible the teacher merely got it wrong (has been known to happen).


    2. Relevant equations

    s=ut + (1/2)at^2



    3. The attempt at a solution

    2dv7hpl.jpg
     
    Last edited: Oct 21, 2011
  2. jcsd
  3. Oct 22, 2011 #2

    SammyS

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    Your equation,  h = 0(t2-t1) + (1/2)a(t2-t1)2  is in error.

    The velocity, u1, at time, t1 is not zero. This velocity is give by:
    u1 = u0 + a(t1-t0) = 0 + a t1 = a t1 .​

    ∴ h = u1(t2-t1) + (1/2)a(t2-t1)2
    = (a t1)(t2-t1) + (1/2)a(t2-t1)2

    If you multiply this all out, combine like terms and then factor it, you will find that your teacher did indeed give the correct answer.

    BTW: There is an easier way to come up with his answer. I thought it better to use what you started with.
     
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