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Equations of motion on exam

  1. May 13, 2012 #1
    In both my Maths and Physics exams they give these equations of motion:

    v = u + at
    v2 = u2 + 2as
    s = ut + (1/2)at2
    s = (u+v/2)t

    However the following equation is never included (even though it can simplify some problems significantly)

    s = vt - (1/2)at2

    Why is this deliberately omitted by the exam boards? Is it because this formula can only be applied in certain situations ?

  2. jcsd
  3. May 13, 2012 #2


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    Staff: Mentor

    What happens if the acceleration a is negative in your third equation? :wink:
  4. May 13, 2012 #3
    from v=u+at, put u=v-at in third.what will you get
  5. May 13, 2012 #4
    Oh then it becomes s = vt + 1/2(at2)? but this is still different to s = ut+1/2(at2) right? Because in the latter equation you would substitute a value for u, but if you do not have u given, then you would use the other one where you substitute a value for v?
  6. May 13, 2012 #5
    It just abbreviation. Different books different abbreviation for initial and final velocity

    vi, vf
    u, v

    Once you adopt u=initial and v=final, be consistent.
  7. May 13, 2012 #6


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    Staff: Mentor

    No, the acceleration is not squared. Only the time is squared. What happens to the sign of the whole term?
  8. May 14, 2012 #7
    Oh sorry, I meant to say :

    but s = ut - (1/2)at2 is not the same as s = vt - (1/2)at2 is it? Because the latter requires you to substitute a value for 'v' (final velocity), whilst the first one require you to substitute a value for 'u' (initial velocity)..
  9. May 14, 2012 #8
    The negative is included when acceleration is negative like gravity which will adjust itself to get positive time since if ur going from high elevation to low elevation you will get positive time when you solve for it. Remember those equations are only used when acceleration is constant not changing.
  10. May 14, 2012 #9
    Your velocity multiplied by "t" is always the initial velocity no matter which letter is used (u or v). Different books (and often different chapters in the same book) use different symbols.
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