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Speed and acceleration

  1. Feb 26, 2008 #1
    1. The problem statement, all variables and given/known data
    A truck covers 45.0 m in 8.50 s while smoothly slowing down to a final speed of 2.20m/s
    a. Find its original speed
    b. find its acceleration


    2. Relevant equations

    average velocity=xf-xi/change in t
    average acceleration=vf-vi/chang in t
    final velocity=vi+at


    3. The attempt at a solution

    ____________________________

    0 8.5 s
    2.20m/s

    x-2.20m/s/(8.5-0s)

    I know that the entire distance is 45.0 m. I don't think I can use any of the three formulas mentioned above, but I'm not sure which other one I could use. Could someone please show me what formula I need or give me a hint, please?

    Thank you very much
     
  2. jcsd
  3. Feb 26, 2008 #2
    Since the truck is "smoothly slowing down", the acceleration must be constant and negative.

    There are many nice formulas involving distance and speed with constant acceleration. Which ones do you know?
     
  4. Feb 26, 2008 #3
    You're on the right track with some of the formulas you wrote. Think kinematics.
     
  5. Feb 26, 2008 #4
    Thank you very much

    I know that vf=vi+at, so vi must be=vf-at

    acceleration=vf-vi/change in time

    Therefore, vi=vf-[(vf-vi)/(change in time)*(t)] right? This doesn't seem to help.

    I know that vf=2.20m/s and the change in time is 8.5. It seems that I need to know what vi or a is. 2.20m/s=vi+a(8.5) or would it just be v=d/change in time? vi=45/8.5=5.3
    vf=vi+at
    2.2=5.3+a(8.5)
    a=-3.65

    Could someone please explain to me how I could find vi, if I don't know what the acceleration is, please?

    Thank you very much
     
    Last edited: Feb 26, 2008
  6. Feb 26, 2008 #5
    You would just use this formula

    d=[tex]\frac{V_i+V_f}{2}[/tex]*t
     
    Last edited: Feb 26, 2008
  7. Feb 26, 2008 #6
    Thank you very much

    Does this look right?

    d=vi+vft/2

    45=vi+2.2m(8.50)/2

    vi=71.3

    a=vf-vi/t

    a=2.2-71.3/8.5
    =-8.13

    Thank you
     
  8. Feb 26, 2008 #7
    sorry didnt realise how the equation came out. You multiply time t by (V_f+V_i)/2
     
  9. Feb 26, 2008 #8
    It's okay. Does this look right now?

    45=8.5vi+18.7/2

    vi=8.34

    a=2.2-8.34/8.5=-.72

    Thank you
     
  10. Feb 26, 2008 #9
    Looks good to me, but I would get another person to check it just in case because I could be wrong.
     
  11. Feb 26, 2008 #10
    Thank you very much

    Regards
     
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