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5 Kinematic Equations?

  1. Sep 12, 2013 #1
    I know that there's 5 key equations for motion which is:

    d = (vf+vi/2)t
    vf = vi + at
    d = vit + 1/2at^2
    vf^2 = vi^2 + 2ad
    d = vft - 1/2at^2

    But my teacher was confusing me today and he taught us these two other equations:

    d = 1/2(vi+vf)t
    d = 1/2at^2

    What are these? Are they just other "rules of motion" like everything else?
  2. jcsd
  3. Sep 12, 2013 #2
    The second equation 'D=1/2at^2' is one of the kinematic equations 'D=Vi+1/2at^2' where the initial velocity 'Vi' is considered to be zero.
  4. Sep 12, 2013 #3
    oh right, i can't believe i didn't notice that! ok thank you!
  5. Sep 12, 2013 #4


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    Staff: Mentor

    This one is incorrect. Either your teacher or you copied it wrong.

    This is the correct version, assuming you meant (1/2)(vi+vf)t.

    Actually, only two equations are essential:

    vf = vi + at
    d = vit + (1/2)at2

    The others can be derived from these two.
  6. Sep 13, 2013 #5
    5 eq

    v final velocity
    u initial vel.
    t time
    s displacement
    a constant accn

    v= u +at
    s= ut + .5at2
    v2= u2 +2as
    s= vt - .5at2
    s= .5(v+u)t
  7. Sep 13, 2013 #6
    but while doing numerical it is irritating to first get acceleration, so actually 5 eqn
    are good adding to it this gives feel to a child what he's doing
    also kinematics it a beginning so one can learn these quickly as afterward formula formula formula!!!
  8. Sep 13, 2013 #7
    You can chose any two out of the five to take as essential and derive the other three from them
  9. Sep 13, 2013 #8


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    Staff: Mentor

    True, but I like those two because if you know calculus you can get them by integrating d2x/dt2 = a twice. Of course, that means you really need to remember only one equation which basically just says "acceleration is constant." :biggrin:
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