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Homework Help: Finding initial speed and accleration

  1. Sep 2, 2007 #1
    1. A truck covers 50.0 m in 9.00 s while smoothly slowing down to final speed 2.40 m/s.

    Find the trucks original speed.

    Find its accleration.


    2. Relevant equations
    im using
    x = v0*t + 1/2 *a*t2
    and
    v2 = v02 + 2*a*x

    I believe this is what the question gives me.

    Xo = 0
    Xf = 50
    Vo = ?
    Vf = 2.4
    a = ?
    t = 9 secs


    3. im positive that i have to use the two equations stated above. but both equations have accleration and Vo as unknown variables. i just need a hint on whats the best way to find accleration or Vo. I dont know if im clear what im saying but its the best way i can explain it. sorry.
     
  2. jcsd
  3. Sep 2, 2007 #2
    Hm...ok I tried the problem (I suck @ sig figs hope you're better @ them).

    first your formula's are wrongish:

    [tex]x=x_0+v_0 t+\frac{1}{2}at^2[/tex]
    [tex]v=v_0+at[/tex]

    you could easily use the 2n'd formula to find a but you don't know initial v.

    So use the first formula, since you're given begining/ending position and ending speed.
    But you need to solve for initial v first. even though you don't know it you know that v=v0+at so move the variable's on the 2n'd formula to solve for initial v, plug that into the first and solve for a, and go from there.
     
  4. Sep 2, 2007 #3
    thanks. worked them to get right answer. silly mistake using wrong equations.
     
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