5 Kinematic Equations?

  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. 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. oh right, i can't believe i didn't notice that! ok thank you!
  5. jtbell

    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. 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. 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. You can chose any two out of the five to take as essential and derive the other three from them
  9. jtbell

    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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